boards: m5stack: cores3: Add bus_5v fixed regulator

Add a `bus_5v` fixed-regulator node driven by the `BOOST_EN` pin. This
enables drivers and shields to reference the 5V supply through the standard
`vin-supply` property, providing correct power sequencing and runtime
control via Zephyr’s regulator API.

This regulator is equivalent to the `bus_5v` already defined on the M5Stack
Core2 board.

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