[ARM] bcm2835: Implement arch_board_reset() & arch_board_shutdown() for Raspberry Pi

This patch does few cleanups and implements missing board functions required for Raspberry Pi.

With this patch, we have Xvisor ARM working on actual Raspberry Pi Mobel B board and Raspberry Pi emulated by QEMU.

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