mb/asus/p5ql-em: Add default value for `gfx_uma_size`

Taken from Asus P5QPL-AM.

Change-Id: If26e98eba5d762d99991bfc06cad1b84e1f430e3
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56562
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asus/p5ql-em/cmos.default

@@ -2,3 +2,4 @@ boot_option=Fallback
 debug_level=Debug
 power_on_after_fail=Disable
 nmi=Enable
+gfx_uma_size=64M