rpi: Determine PL011/Mini-UART availability at runtime

Firmware on the Raspberry Pi family of devices can dynamically configure either
the PL011, Mini-UART or no device at all to be routed to the user accessible
UART pins.

That means we need to always include both drivers, because we can never be sure
which of the two serial devices firmware actually chooses to use.

Signed-off-by: Alexander Graf <agraf@suse.de>