powerpc: doc/dts-bindings: document mpc5121 psc uart dts-bindings

Support for MPC5121 PSC UART in the mpc52xx_uart driver
added new DTS properties for FSL MPC5121 PSC FIFO Controller.
Provide documentation of the new properties and some examples.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>