OvmfPkg: QemuFlashFvbServicesRuntimeDxe: fix VALID_ARCHITECTURES in INF

We build this driver for X64 as well -- the comment isn't overly
important, but it shouldn't be misleading.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18667 6f19259b-4bc3-4df7-8a09-765794883524