[POWERPC] 8xx: Add clock-frequency to adder875 and mpc885ads dts

cpm_uart_core has a dependency on fsl,cpm-brg/clock-frequency, this
means that a .dts that uses the cpm uart driver needs to supply a
clock-frequency entry for get_brgfreq to return a meaningful number.

Signed-off-by: Bryan O'Donoghue <[e-mail]>
Signed-off-by: Kumar Gala <[e-mail]>

arch/powerpc/boot/dts/adder875-redboot.dts

···
 				compatible = "fsl,mpc875-brg",
 				             "fsl,cpm1-brg",
 				             "fsl,cpm-brg";
+				clock-frequency = <50000000>;
 				reg = <0x9f0 0x10>;
 			};
 

arch/powerpc/boot/dts/adder875-uboot.dts

···
 				compatible = "fsl,mpc875-brg",
 				             "fsl,cpm1-brg",
 				             "fsl,cpm-brg";
+				clock-frequency = <50000000>;
 				reg = <0x9f0 0x10>;
 			};
 

arch/powerpc/boot/dts/mpc885ads.dts

···
 				compatible = "fsl,mpc885-brg",
 				             "fsl,cpm1-brg",
 				             "fsl,cpm-brg";
+				clock-frequency = <0>;
 				reg = <9f0 10>;
 			};