mips: bmips: fix BCM3380 periph clock frequency

Instead of having a peripheral clock of 50 MHz like the BCM63xx family, it
has a 48 MHz clock.
This fixes uart baud rate calculation for BCM3380.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>

arch/mips/dts/brcm,bcm3380.dtsi

@@ -42,7 +42,7 @@
 		periph_osc: periph-osc {
 			compatible = "fixed-clock";
 			#clock-cells = <0>;
-			clock-frequency = <50000000>;
+			clock-frequency = <48000000>;
 			u-boot,dm-pre-reloc;
 		};