ARM: shmobile: Check r8a7791 MD21 at SMP boot

On r8a7791 the hardware boot mode bit MD21 indicates if hardware
debug mode is enabled or not. In case hardware debug mode is enabled
print a warning and refrain from booting secondary CPU cores.

Without this patch Koelsch with SW8-4 set to OFF will hang at SMP boot.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>