arm: implement find_next_zero_bit function

This commit copies implementation of the find_next_zero_bit() from
git://git.denx.de/u-boot.git/arch/mips/include/asm/bitops.h

. v2014.07

The function is required to enable MCAST_TFTP support for ARM platforms.

Signed-off-by: Vitaly Andrianov <vitalya@ti.com>