drivers: led: lp5569: add charge pump configuration

The lp5569 controller contains an internal charge pump which can be useful
for driving LEDs with a forward voltage greater than the lp5569 supply.
Taking advantage of the charge pump can alleviate the need for an external
boost converter.

Add a dts property, charge-pump-mode, to the ti,lp5569 binding with which
the cp_mode bits of the MISC register will be configured.

Following the datasheet, the possible configurations are:
    0x00 -> disabled (default)
    0x01 -> 1x mode
    0x10 -> 1.5x mode
    0x11 -> auto mode

Signed-off-by: Emil Juhl <emdj@bang-olufsen.dk>