acpi: sync FADT flags from PIIX4 to Q35

The following commits updated the FADT flags for PIIX4:
commit c7be281b3be0f2013aa4b5d5f400981a1adb6e63
    Drop FIX_RTC flag from FADT.
commit a4e2b9adbba95c2d783761e84444bbb3c8d6107b
    enable USE_PLATFORM_CLOCK bit in FADT flags

Q35 support was originally written before these commits in seabios, PIIX
got updated, Q35 didn't.  QEMU uses the same clock for both so there's
no reason for these bits to differ.  Sync them up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>