target/arm: New helper function arm_v7m_mmu_idx_all()

target/arm: New helper function arm_v7m_mmu_idx_all()

Add a new helper function which returns the MMU index to use
for v7M, where the caller specifies all of the security
state, privilege level and whether the execution priority
is negative, and reimplement the existing
arm_v7m_mmu_idx_for_secstate_and_priv() in terms of it.

We are going to need this for the lazy-FP-stacking code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190416125744.27770-21-peter.maydell@linaro.org