sunxi-leds: Add leds_trigger_# fex parameter

Allows the default trigger to be specificed in the .fex file.

Example use-case
To let a LED light up on SD card activity create a script.fex section like this:

==
[leds_para]
leds_used = 1
leds_num = 1
leds_pin_1 = port:PH20<1><default><default><0>
leds_name_1 = "ph20:green:led1"
leds_trigger_1 = "mmc0"
==

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>