BCM270X_DT: Add umbrella I2C Mux overlay i2c-mux

This overlay supports a range of I2C multiplexers - PCA9542 (2 ports),
PCA9545 (4 ports) and PCA9548 (8 ports).

Also remove the dedicated i2c-mux-9548a overlays since it is no longer
needed.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>