[ARM] pxa: introduce cpu_is_pxa26x()

PXA26x (PXA261/262) is actually a PXA250 with stacked Intel(R)
StartaFlash. And this can be decided by bit 3 (PKG_TYPE) of
BOOT_DEF register.

Due to this extra I/O register access, make cpu_is_pxa26x() a
public function instead of a macro.

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