dt-bindings: thermal: rockchip: document otp thermal trim

Several Rockchip SoCs, such as the RK3576, can store calibration trim
data for thermal sensors in OTP cells. This capability should be
documented.

Such a rockchip thermal sensor may reference cell handles that store
both a chip-wide trim for all the sensors, as well as cell handles
for each individual sensor channel pointing to that specific sensor's
trim value.

Additionally, the thermal sensor may optionally reference cells which
store the base in terms of degrees celsius and decicelsius that the trim
is relative to.

Each SoC that implements this appears to have a slightly different
combination of chip-wide trim, base, base fractional part and
per-channel trim, so which ones do which is documented in the bindings.

Signed-off-by: Nicolas Frattaroli <nicolas.frattaroli@collabora.com>