serial: uartlite: move from byte accesses to word accesses

Byte accesses for I/O devices in Xilinx IP is going to be less
desired in the future such that the driver is being changed to
use 32 bit accesses.

This change facilitates using the uartlite IP over a PCIe bus
which only allows 32 bit accesses.

Signed-off-by: John Linn <john.linn@xilinx.com>
Tested-by: Michal Simek <monstr@monstr.eu>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>