armv7m_nvic: fix CPUID Base Register

cp15.c0_cpuid is never initialized for ARMv7-M; take the value directly
from cpu->midr instead.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Message-id: 1398036308-32166-1-git-send-email-rabin@rab.in
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
