avr32: add generic_find_next_le_bit bit function

This patch implements the generic_find_next_le_bit bit function for AVR32
architecture. This is used by EXT4 file system.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>