[ARM] arm32ve: Save/restore MMU context for long descriptor format

In case of long descriptor format (i.e. MMU LPAE), the TTBR0/1 registers
are 64bit and PRRR/NMRR are treated as MAIR0/1 registers.

This patch ensures that we save/restore MMU context for both short &
long descriptor formats.

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