Platform/RPi3: Disable DEBUG output at runtime for the RTC driver

Using DEBUG () at runtime can crash the OS if we don't take care to
inhibit serial output to MMIO mapped UARTs that we don't inform the
OS about via the memory map.

So use a version of DebugLib that does exactly that for the RTC
driver.

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