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: 0de4ab81ab26 ("ARM: dts: imx6dl-yapp4: Add Y Soft IOTA Crux/Crux+ board")
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Petr Benes <[e-mail]>
Signed-off-by: Michal Vokáč <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6q-yapp4-crux.dts

···
 	status = "okay";
 };
 
+&reg_pu {
+	regulator-always-on;
+};
+
 &reg_usb_h1_vbus {
 	status = "okay";
 };

arch/arm/boot/dts/imx6qp-yapp4-crux-plus.dts

···
 	status = "okay";
 };
 
+&reg_pu {
+	regulator-always-on;
+};
+
 &reg_usb_h1_vbus {
 	status = "okay";
 };