Platform/SbsaQemu: Switch to VirtNorFlashDxe

Switch to the OVMF version of the NOR flash DXE driver, which supports
QEMU's NOR flash emulation specifically, and carries some optimizations
that are therefore permitted.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>