[POWERPC] Uartlite: Flush RX fifo in bootwrapper

Flush the uartlite RX fifo so that characters typed before entry into
the zImage wrapper do not muck up the kernel command line.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>