ARM: shmobile: r8a7791: Allow booting secondary CPU cores in debug mode

Now debug resource reset is handled properly, allow booting secondary
CPU cores when hardware debug mode is enabled (MD21=1, SW8-4=OFF on
koelsch) on legacy r8a7791.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>