ARM: pxa: fix mfpr_sync to read from valid offset

Since mfpr_mmio_base[0] is not always valid on later SoCs,
fixed mpfr_sync() to read back from valid mfp offset always.

Signed-off-by: Yu Tang <ytang5@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>