target/arm: Stub arm_hcr_el2_eff for m-profile

target/arm: Stub arm_hcr_el2_eff for m-profile

M-profile doesn't have HCR_EL2.  While we could test features
before each call, zero is a generally safe return value to
disable the code in the caller.  This test is required to
avoid an assert in arm_is_secure_below_el3.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20230227225832.816605-3-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>