MIPS: AR7: Constify gpio_led

gpio_leds are not supposed to change at runtime. struct
gpio_led_platform_data contains a const struct gpio_led pointer since
v2.6.39, so mark the gpio_led structures const too.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18007/


[jhogan@kernel.org: improve commit message]
Signed-off-by: James Hogan <jhogan@kernel.org>