stm32mp1: activate serial rx buffer

Activate the serial rx buffer.
Prepare console MUX activation with vidconsole, and avoid console
performance issue (missing character for copy-paste).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/stm32mp15_basic_defconfig b/configs/stm32mp15_basic_defconfig
index ed82a0a..e134a66 100644
--- a/configs/stm32mp15_basic_defconfig
+++ b/configs/stm32mp15_basic_defconfig
@@ -45,4 +45,5 @@
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_DM_REGULATOR_STM32_VREFBUF=y
 CONFIG_DM_REGULATOR_STPMU1=y
+CONFIG_SERIAL_RX_BUFFER=y
 CONFIG_STM32_SERIAL=y