[ARM] 4526/1: pxa: make ARCH_PXA select GENERIC_CLOCKEVENTS

make ARCH_PXA select GENERIC_CLOCKEVENTS to fix the
building issue due to merge of clockevents based PXA
timer

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