drivers: led: introduce channel-dedicated syscalls

Some LED controllers are connected to a large number of LEDs (i.e.
array/strip of LEDs). A user may need to set several LEDs at the same
time. The LED oriented syscalls are not adapted to this task. Indeed a
call per LED is needed to configure a group of LEDs. To that end, this
patch adds the led_set_channel and led_write_channels syscalls to the
LED API. They offer direct access to the raw channels. They respectively
allow to set a single channel and to write several contiguous channels
(and therefore to configure a group of LEDs).

Moreover the addition of the led_write_channels syscall also aims at
closing the gap with the LED strip API.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>
Signed-off-by: Maxime Bittan <maxime.bittan@seagate.com>