ArmPlatformPkg/ArmVExpressPkg: Add Shutdown support

ArmPlatformPkg/ArmVExpressPkg: Add Shutdown support

LibResetSystem was performing a cold reboot for both reboot and
shutdown.

This patch updates LibResetSystem to pass SYS_CFG_SHUTDOWN to the
microcontroller when called wit EfiResetShutdown.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14438 6f19259b-4bc3-4df7-8a09-765794883524