boards: m5stack: cores3: Add bus_out and usb_otg fixed regulators

Add two fixed-regulator nodes to model the load switches on the M5Stack
CoreS3 that control 5V output to the M-Bus/Grove connectors and the USB
VBUS line.

- `bus_out_reg`: controls the BUS_OUT 5V rail (BUS_OUT_EN).
- `usb_otg_reg`: controls USB VBUS output (USB_OTG_EN).

The BUS_OUT regulator is enabled by default to match the behavior of
existing M5Stack boards. USB_OTG remains disabled by default, since
ESP32-S3 does not currently support USB host mode.

Signed-off-by: Chen Xingyu <hi@xingrz.me>