ArmPlatformPkg/PL031RealTimeClockLib: ignore DST setting when timezone is set

ArmPlatformPkg/PL031RealTimeClockLib: ignore DST setting when timezone is set

According to the UEFI spec, the timezone setting which the platform needs
to record in addition to the actual date and time already reflects the
current DST setting. In other words, moving the clock from standard time
to daylight saving time also involves adding or subtracting 60 minutes
from the timezone setting, as well as flicking the EFI_TIME_IN_DAYLIGHT
bit in the DST setting.

This means we need to disregard the DST setting if the timezone is
specified, and only add or subtract the additional hour if we are on
local time.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>