leds: mlxreg: Add support for capability register

Add support for capability register in order to distinct between the
systems with minor LED configuration differences. It reduces the amount
of code describing systems' LED configuration.
For example one system can be equipped with six LED, while the other
with only four. Reading this information from the capability registers
allows to use the same LED structure for such systems and set the
relevant configuration dynamically based on capability register
content.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>