[ARM] pxa: make cpu_is_pxa2* macros more consistent

1. add a CPUID table in the comment

2. make cpu_is_pxa25x() true for PXA210/250/255/26x

3. PXA210 is treated as PXA25x, all related code modified to
   reflect this

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