[TESTS] Use SRAM for basic firmware on VExpress-A9 & VExpress-A15 guests

We have 32MB SRAM available for both VExpress-A9 and VExpress-A15 guests
hence we can use this SRAM for basic firmware instead of running basic
firmware from normal RAM.

Signed-off-by: Anup Patel <anup@brainfault.org>