ARM: dts: imx6dl-yapp4: Do not allow PM to switch PU regulator off on Q/QP

Fix our design flaw in supply voltage distribution on the Quad and QuadPlus
based boards.

The problem is that we supply the SoC cache (VDD_CACHE_CAP) from VDD_PU
instead of VDD_SOC. The VDD_PU internal regulator can be disabled by PM
if VPU or GPU is not used. If that happens the system freezes. To prevent
that configure the reg_pu regulator to be always on.

Fixes: 0de4ab81 ("ARM: dts: imx6dl-yapp4: Add Y Soft IOTA Crux/Crux+ board")
Cc: petrben@gmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Petr Benes <petr.benes@ysoft.com>
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>