[MIPS] Alchemy: Register platform devices

This patch separates the platform devices registration for the MTX-1
specific devices: GPIO leds and watchdog.

[Minor fixup and formatting change -- Ralf]

Signed-off-by: Florian Fainelli <florian.fainelli@telecomint.eu>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>