leds: leds-ns2: depends on MACH_ARMADA_370

The leds-ns2 driver is also used by the n090401 board (Seagate NAS
4-Bay), which is based on the Marvell Armada-370 SoC.

Then this patch allows to select the leds-ns2 driver if MACH_ARMADA_370
is enabled. Additionally, this also updates the Kconfig help message.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>