Platform/RPi: Inhibit serial output for all runtime drivers

This is an improvement of e9db0463
where we inhibit serial output of MMIO mapped UARTs to all runtime
drivers rather than just RTC, as other drivers may crash the OS
just the same.

Also add it to the Pi 4 platform where it was missing altogether.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>