amba_pl011: Insert mb() for correct FIFO handling

The pl011 register accessor functions use the _relaxed versions of the
standard readl() and writel() functions, meaning that there are no
automatic memory barriers. When polling a FIFO status register to check
for fullness, it is necessary to ensure that any outstanding writes have
completed; otherwise the flags are effectively stale, making it possible
that the next write is to a full FIFO.

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