dt-bindings: mfd: lp87565: Convert to yaml

Also apply a few smaller improvements:

 - document that the only possible I2C slave address is 0x60 as per the
   datasheet and fix the second example accordingly
 - The definition of "xxx-in-supply" was generic, thus define in detail the
   possible cases for each chip variant
 - remove "MFD driver" from the title as it is implementation specific
 - replace "PMIC" with "buck converter" in title as it is more informative

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>