arm: ep93xx: use gpio_led_register_device

Use gpio_led_register_device to register the two leds
connected to the ep93xx.

Since this is a common feature for all ep93xx based boards,
create a new Kconfig option, EP93XX_SOC_COMMON, to select
the LEDS_GPIO_REGISTER feature.

The struct gpio_led and struct gpio_led_platform_data variables
have been changed to const struct because of:

commit 9517f925
	leds: make *struct gpio_led_platform_data.leds const

They have also been marked as __initconst because the helper
function gpio_led_register_device makes a deep copy of the
platform data to allow it to live in init memory:

commit 4440673a
	leds: provide helper to register "leds-gpio" devices

Signed-off-by: Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ryan Mallon <rmallon@gmail.com>