[ARM] pxa: add PXA3 cpu_is_xxx() macros

Extracted from patch by Eric Miao, this adds the cpu_is_xxx() macros
for identifying PXA3 SoCs.

Signed-off-by: eric miao <eric.y.miao@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>