MIPS: Malta: Use PIIX4 poweroff driver to power down

Remove the platform code used to power down the system, instead relying
upon the new PIIX4 poweroff driver. This reduces the amount of platform
code required for the Malta board in preparation for allowing it to be
part of a more generic kernel.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/14282/