ifdtool: Add SPI_FREQUENCY_50MHZ_30MHZ as a valid freq

Without this change, error "Unknown descriptor version: 4" will be
returned if this frequency is selected (seen on GLKRVP)

Change-Id: Ib5bfb996b85c7245d8f9c70988bfd5bbac882d74
Signed-off-by: Hannah Williams <hannah.williams@intel.com>
Reviewed-on: https://review.coreboot.org/18688
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

util/ifdtool/ifdtool.c

@@ -90,6 +90,7 @@ static void check_ifd_version(char *image, int size)
 		max_regions = MAX_REGIONS_OLD;
 		break;
 	case SPI_FREQUENCY_17MHZ:
+	case SPI_FREQUENCY_50MHZ_30MHZ:
 		ifd_version = IFD_VERSION_2;
 		max_regions = MAX_REGIONS;
 		break;