hw/misc/aspeed_scu: Handle AST2600 protection key registers correctly

hw/misc/aspeed_scu: Handle AST2600 protection key registers correctly

The AST2600 SCU has two protection key registers (0x00 and 0x10) that
both need to be unlocked. (Un-)locking 0x00 modifies both protection key
registers, while modifying 0x10 only modifies itself.

This commit updates the SCU write logic to reject writes unless both
protection key registers are unlocked, matching the behaviour of
real hardware.

Signed-off-by: Tan Siewert <tan@siewert.io>
Reviewed-by: Jamin Lin <jamin_lin@aspeedtech.com>
Link: https://lore.kernel.org/qemu-devel/20250619085329.42125-1-tan@siewert.io
Signed-off-by: Cédric Le Goater <clg@redhat.com>