Define global uma_memory variables

Use of the uma_memory_base and _size variables is very scattered.
Implementation of setup_uma_memory() will appear in each northbridge.

It should be possible to do this setup entirely in northbridge
code and get rid of the globals in a follow-up.

Change-Id: I07ccd98c55a6bcaa8294ad9704b88d7afb341456
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/1204
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

src/southbridge/amd/rs780/rs780.c

@@ -221,7 +221,6 @@ static void rs780_nb_gfx_dev_table(device_t nb_dev, device_t dev)
 	/* Program Straps. */
 	romstrap2 = 1 << 26; // enables audio function
 #if CONFIG_GFXUMA
-	extern uint64_t uma_memory_size;
 	// bits 7-9: aperture size
 	// 0-7: 128mb, 256mb, 64mb, 32mb, 512mb, 1g, 2g, 4g
 	if (uma_memory_size == 0x02000000) romstrap2 |= 3 << 7;