[TESTS] Use SRAM from secondary spin in ARM fast model bootwrapper

Currently, we use normal RAM address 0x84000000 for secondary spin
when compiling ARM fast model bootwrapper.

Since the secondary spin is in RAM we always face this risk of
some Xvisor code tamperring that RAM location while the secondary
CPUs spining from this RAM location.

The VExpress-A15 SRAM is not used currently hence we use SRAM for
secondary spin when compiling ARM fast model bootwrapper.

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