hw/intc/armv7m_nvic: Allow reading of M-profile MVFR* registers

hw/intc/armv7m_nvic: Allow reading of M-profile MVFR* registers

For M-profile the MVFR* ID registers are memory mapped, in the
range we implement via the NVIC. Allow them to be read.
(If the CPU has no FPU, these registers are defined to be RAZ.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-3-peter.maydell@linaro.org