ArmPlatformPkg/PL031RealTimeClockLib: cast EfiTimeToEpoch() val. to UINT32

ArmPlatformPkg/PL031RealTimeClockLib: cast EfiTimeToEpoch() val. to UINT32

In preparation for changing EfiTimeToEpoch()'s return type to UINTN, cast
EfiTimeToEpoch()'s retval to UINT32 explicitly, in LibSetTime().

Currently, this is a no-op, and even after widening the retval, it will
make no difference, as LibSetTime() explicitly restricts Time->Year under
2106, given that "the PL031 is a 32-bit counter counting seconds". The
patch is made for preventing compiler warnings.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20201221113657.6779-2-lersek@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>