ARM: pxa: fix cpu_is_pxa*() not expanding to zero when not configured

When CONFIG_PXA3xx is not selected, cpu_is_pxa3xx() doesn't expand to
zero, which in some places doesn't result in correct optimization.

Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Reported-and-tested-by: Marek Vasut <marek.vasut@gmail.com>