leds: ns2: convert to fwnode API

Convert from OF api to fwnode API, so that it is possible to bind this
driver without device-tree.

The fwnode API does not expose a function to read a specific element of
an array. We therefore change the types of the ns2_led_modval structure
so that we can read the whole modval array with one fwnode call.

Signed-off-by: Marek Behún <kabel@kernel.org>
Tested-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>