Merge common code for RasPi 3 and 4

[axel-h]

Seems the Raspi 3 and 4 basically use the same timer. And some other peripherals look quite identical, too. Once the RasPi4 support is merged, we should look into reducing the amount of code duplication. Instead, factor the common parts out into a shared place (e.g. libplatsupport/src/mach/bcm) like we do int with zynq or exynos