leds: netxbig: remove legacy board-file support

Since commit ebc278f1 ("ARM: mvebu: remove static LED setup for
netxbig boards"), no one in upstream passes in the platform data to
this driver.

Squash leds-kirkwood-netxbig.h into the driver, and remove the legacy
board-file support.

Link: https://lkml.org/lkml/2019/7/20/83


Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>