serial: add BCM283x mini UART driver

The RPi3 typically uses the regular UART for high-speed communication with
the Bluetooth device, leaving us the mini UART to use for the serial
console. Add support for this UART so we can use it.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
diff --git a/drivers/serial/Makefile b/drivers/serial/Makefile
index 05bdf56..ee7147a 100644
--- a/drivers/serial/Makefile
+++ b/drivers/serial/Makefile
@@ -38,6 +38,7 @@
 obj-$(CONFIG_STM32_SERIAL) += serial_stm32.o
 obj-$(CONFIG_PIC32_SERIAL) += serial_pic32.o
 obj-$(CONFIG_STM32X7_SERIAL) += serial_stm32x7.o
+obj-$(CONFIG_BCM283X_MU_SERIAL) += serial_bcm283x_mu.o
 
 ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_USB_TTY) += usbtty.o