Add hwmon support to the ZL3073x DPLL driver, exposing die temperature
and input reference frequency measurements via the standard hwmon
interface.

Patch 1 registers an hwmon device with the standard temperature sensor
channel. The die temperature is only available on chip variants with the
ZL3073X_FLAG_DIE_TEMP flag and visibility is controlled via the
is_visible callback. The temperature register value in 0.1°C units is
converted to millidegrees Celsius.

Patch 2 adds the frequency measurement infrastructure. It extracts
common measurement latch logic into a zl3073x_ref_freq_meas_latch()
helper shared with the existing FFO measurement code, and adds
zl3073x_ref_freq_meas_update() to periodically read absolute input
reference frequencies in Hz.

Patch 3 exposes the measured frequencies via custom sysfs attributes
(freqN_input and freqN_label) since hwmon has no native frequency
sensor type. A cached device ready flag ensures freq_input_show()
returns -ENODATA without I2C access when firmware is not configured.

The hwmon registration is conditionally compiled using
IS_REACHABLE(CONFIG_HWMON) with a stub fallback.

Tested on Microchip EDS2 (pcb8385) platform:

  $ sensors zl3073x-i2c-1-70
  zl3073x-i2c-1-70
  Adapter: AT91
  temp1:        +24.2°C
  REF0P:        125 MHz
  REF0N:          0 Hz
  REF1P:        125 MHz
  REF1N:        125 MHz
  REF2P:         10 MHz
  REF2N:       1000 mHz
  REF3P:          0 Hz
  REF3N:       1000 mHz
  REF4P:         25 MHz
  REF4N:        170 kHz

  $ sensors -u zl3073x-i2c-1-70
  zl3073x-i2c-1-70
  Adapter: AT91
  temp1:
    temp1_input: 24.200
  REF0P:
    freq0_input: 124999220.000
  REF0N:
    freq1_input: 0.000
  REF1P:
    freq2_input: 125001660.000
  REF1N:
    freq3_input: 125001661.000
  REF2P:
    freq4_input: 10000000.000
  REF2N:
    freq5_input: 1.000
  REF3P:
    freq6_input: 0.000
  REF3N:
    freq7_input: 1.000
  REF4P:
    freq8_input: 25000000.000
  REF4N:
    freq9_input: 169598.000

Ivan Vecera (3):
  dpll: zl3073x: add hwmon support for die temperature
  dpll: zl3073x: add input reference frequency measurement
  dpll: zl3073x: add hwmon support for input reference frequencies

 drivers/dpll/zl3073x/Makefile |   1 +
 drivers/dpll/zl3073x/core.c   |  91 ++++++++++++++++----
 drivers/dpll/zl3073x/core.h   |   2 +
 drivers/dpll/zl3073x/hwmon.c  | 151 ++++++++++++++++++++++++++++++++++
 drivers/dpll/zl3073x/hwmon.h  |  16 ++++
 drivers/dpll/zl3073x/ref.h    |  14 ++++
 6 files changed, 261 insertions(+), 14 deletions(-)
 create mode 100644 drivers/dpll/zl3073x/hwmon.c
 create mode 100644 drivers/dpll/zl3073x/hwmon.h

-- 
2.52.0