ARM: shmobile: Check r8a7791 MD21 at SMP boot

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>
(cherry picked from commit 277efd30cfc72ec2f44a9bc95d93807b867bd9e9)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>