gm45: enable setting all vram sizes from cmos

Setting the size of the preallocated memory for the igd is done
using a cmos parameter, gfx_uma_size. This was limited to a subset of
all available sizes, that were already implemented elsewhere
in the northbridge code.

What this does is change the cmos parameter to 4 bits instead
of 3 bits to accomodate all vram sizes.
It also adds a sane default of 32mb that already was in place.
The northbridge code that reads this cmos parameter is
also changed for this new cmos settings.

352M is disabled since it causes issues on systems with 4GB or more ram.

TEST: Build, flash target. Clear cmos by corrupting
the checksum (nvramtool -c something).
Set a desired value in gfx_uma_size using nvramtool.
"dmesg | grep stolen" to see what is actually allocated.

Change-Id: Ia6479d03f1abe6d0c94bd7264365505e8f8eaeec
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/14900
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

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