serial: xilinx_uartps: Get clock rate info from dts

Add support for specifying clock information for the uart clk via the
device tree. This eliminates the need to hardcode rates in the device
tree.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>