BCM270X_DT: Set spidev spi-max-frequency to 125MHz

The BCM2835 SPI controllers have a maximum bus clock of half the system
clock speed, so with the 250MHz system clock found on Raspberry Pis
you get a theoretical maximum bus speed of 125MHz. Note that this
speed is unlikely to be reliable, and the maximum usable bus spee will
depend on both the attached device and the wiring.

See: https://github.com/raspberrypi/linux/issues/2165


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