realtek: add fan controller support to D-Link DGS-1210-28MP

realtek: add fan controller support to D-Link DGS-1210-28MP

The DGS-1210-28MP has a LM63 fan controller connected via i2c of the
RTL8231. The clock line is always low if the property
i2c-gpio,scl-open-drain is not set; with this property, the GPIO pin is
force-drive and the clock works as expected.

The LM63 is not configured by U-Boot, thus only manual fan control is
possible by settings pwm1_enable to "1" and writing the desired values to
pwm1.

The OEM firmware drives the fan from user mode and sets it up like this:

// PWM LUT/value r/w, PWM Clock = 1.4kHz
0x4a 0x28
// Tachometer spinup disabled, spin-up cycles bypassed
0x4b 0x00
// PWM Frequency = default
0x4d 0x17
// PWM Value (28)
0x4c 0x1c
// If > 0 C, use
0x50 0x00
// PWM = 28
0x51 0x1c
// If > 51 C, use
0x52 0x33
// PWM = 44
0x53 0x2e
// Set hysteresis to 100 = default
0x4f 0x03
// Turn on automatic mode and w/p the LUT values
0x4a 0x08

A thread in the OEM firmware polls the ALERT status register for fan
failures.

Unfortunately, the lm63 kernel driver does not perform any initialization
of the chip and it does not support changing some config registers (like
PWM frequency or LUT). Hence, we are stuck with the defaults and need to do
fan control in software.

Signed-off-by: Andreas Böhler <dev@aboehler.at>
Link: https://github.com/openwrt/openwrt/pull/15616
Signed-off-by: Sander Vanheule <sander@svanheule.net>