ARM: 5772/1: Use REV and REV16 for byte swapping on ARMv6+

ARMv6 introduced the REV and REV16 instructions that reverse bytes in
words and halfwords.  Use them for the arch-specific implementation of
the byte swapping helpers on ARMv6+.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>