MIPS: boot: Fixes for compressed/uart-16550.c

Fix uart-16550.c for adding XLR/XLP support, changes are:
* Make register read/write use volatile pointers
* Support 32 bit IO read/write
* Increase timeout in waiting for UART LSR

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5416/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>