arm: vf610: fix boot from SD-card

Boot from SD-card (and probably also from NAND) was broken since
commit d6d07a9b ("arm: vf610: add NAND support for vf610twr").
It looks like the increased size of U-Boot lead to a situation
where the boot ROM overwrote its own stack/heap while loading
U-Boot from the SD-card to the SRAM. However, U-Boot worked fine
when loaded through USB serial loader directly into SRAM. It
looks like loading from SD-card uses other stack/heap location
then the serial loader (or maybe no stack or heap at all).
This fix moves U-Boot to gfxRAM, which is 512kB in size and is not
used by the boot ROM nor the SD-card loader of it.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Bill Pringlemeir <bpringlemeir@nbsps.com>