Convert CONFIG_SYS_CBSIZE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_CBSIZE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/include/configs/imx8mp_rsb3720.h b/include/configs/imx8mp_rsb3720.h
index d643532..bc0d297 100644
--- a/include/configs/imx8mp_rsb3720.h
+++ b/include/configs/imx8mp_rsb3720.h
@@ -168,8 +168,8 @@
 #define PHYS_SDRAM_2_SIZE		0x80000000	/* 2 GB */
 #endif
 
-/* Monitor Command Prompt */
-#define CONFIG_SYS_CBSIZE		2048
+#define CONFIG_MXC_UART_BASE		UART3_BASE_ADDR
+
 #define CONFIG_SYS_BARGSIZE CONFIG_SYS_CBSIZE
 
 #define CONFIG_SYS_FSL_USDHC_NUM	2