[ARM] Move board specific reset to drivers/power/reset

This patch moves poweroff and reset related code to drivers/power/reset
so that we can further reduce board specific code and make board more
closer to generic board.

More specifically this patch adds power-off/reset driver for BCM2835,
Realview, VExpress, and Sun4i under directory drivers/power/reset
and removes related code from arch/arm/board directory.

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