ARM: dts: bcm271x: Use a53 pmu, drop RPI364

The upstream bcm2837.dtsi uses cortex-a53-pmu, so we can do the same
but with a fallback to the cortex-a7-pmu which is supported by the
32-bit kernel.

Now that we're using the natural fallback mechanism of compatible
strings, the RPI364 macro no longer serves any purpose - remove it.

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