In debug mode (MD21=1), reset requests derived from power-shutoff to the
AP-system CPU cores must be enabled before the AP-system CPU cores
resume from power-shutoff for the first time. Else resume may fail,
causing the system to hang during boot.

As setting these bits is a no-op in normal mode, there's no need to
check the actual state of MD21 first.

Inspired by CPU-specific patches in the BSP by Hisashi Nakamura
<hisashi.nakamura.ak@renesas.com>.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Hiep Cao Minh <cm-hiep@jinso.co.jp>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>