ARM: mmp: fix cpuid detection on mmp2

Fix typo error on cpu_is_mmp2(). Correct cpu_readid_id() to
read_cpuid_id().  Append missing parenthesis.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>