arm: vexpress: Move the poweroff/restart code to drivers/power/reset

This patch moves the arch/arm/mach-vexpress/reset.c functionality to
drivers/platform/reset/ and adds the necessary Kconfig wiring.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Pawel Moll <pawel.moll@arm.com>