arm64: dts: imx8mn-var-som-symphony: add vcc supply for PCA9534

The following warning is shown when probing device:

    pca953x 1-0020: supply vcc not found, using dummy regulator

Define a new fixed 3.3v regulator for carrier board peripherals,
enabled by mosfet switch Q2 after the SOM_3V3 supply rises (no software
control).

Add this new regulator as vcc supply to the PCA9534 to silence the warning.

Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>