[ARM] arm32: Ensure that we are in SVC mode before turning-on MMU

Latest ARMv7 bootloaders will enter Xvisor in HYP-mode if
Virtualization extenstions are available in underlying host HW
as-per Linux ARM32 booting protocol.

This mean if we try to boot Xvisor ARM32 (without virtualization)
on host HW having virtualization extension then bootloader will
jump into Xvisor in HYP-mode. To tackel this, we force-fully switch
to SVC mode before enabling MMU on each host CPU.

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