serial: xuartps: Fix register space size.

The register space size is 0x1000, and this value [not 0xfff] should be
provided to request_mem_region(), ioremap(), etc.

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>