arm64: dts: imx8mp-var-som: Fix LDO5 shutdown causing SD card timeout

Fix SD card timeout issue caused by LDO5 regulator getting disabled
after boot.

The kernel log shows LDO5 being disabled, which leads to a timeout
on USDHC2:
[   33.760561] LDO5: disabling
[   81.119861] mmc1: Timeout waiting for hardware interrupt.

To prevent this, set regulator-boot-on and regulator-always-on for
LDO5. Also add the vqmmc regulator to properly support 1.8V/3.3V
signaling for USDHC2 using a GPIO-controlled regulator.

Fixes: 6c2a1f4f ("arm64: dts: imx8mp-var-som-symphony: Add Variscite Symphony board and VAR-SOM-MX8MP SoM")
Signed-off-by: Himanshu Bhavani <himanshu.bhavani@siliconsignals.io>
Acked-by: Tarang Raval <tarang.raval@siliconsignals.io>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>