[EMULATORS] l2x0: Ignore writes to all lockdown registers

The l2x0 emulates PL310, PL020, and PL021. Among these, PL310
have 16 lock-down registers. This patch extends l2x0 to
ignore all 16 lock-down registers of PL310.

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