ARM: 8443/1: Adding support for atomic half word exchange

Since support for half-word atomic exchange was not there and Qspinlock
on ARM requires it, modified __xchg() to add support for that as well.
ARMv6 and lower does not support ldrex{b,h} so, added a guard code to
prevent build breaks.

Signed-off-by: Sarbojit Ganguly <ganguly.s@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>