ArmPlatformPkg: Actually disable PL031 interrupts

ArmPlatformPkg: Actually disable PL031 interrupts

The PL031 interrupt mask register (IMSC) is not very clearly documented
in the PL031 specification. However, bit 0 (RTCIMSC) indicates whether
interrupts are enabled, not disabled.

So before this commit, we were actually *enabling* interrupts for the RTC.

This patch changes the logic to instead disable interrupts when they
are not disabled already.

Signed-off-by: Alexander Graf <graf@amazon.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>