serial: xilinx_uartps: kill CONFIG_OF conditional

The Zynq platform requires the use of CONFIG_OF.  Remove the #ifdef
conditionals in the uartps driver.  Make dependency explicit in Kconfig.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Tested-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>