arm64: defconfig: enable ARCH_STM32 and STM32 serial driver

Allow a basic boot on STM32MP257 SoC.
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

arch/arm64/configs/defconfig

@@ -58,6 +58,7 @@ CONFIG_ARCH_RENESAS=y
 CONFIG_ARCH_ROCKCHIP=y
 CONFIG_ARCH_SEATTLE=y
 CONFIG_ARCH_INTEL_SOCFPGA=y
+CONFIG_ARCH_STM32=y
 CONFIG_ARCH_SYNQUACER=y
 CONFIG_ARCH_TEGRA=y
 CONFIG_ARCH_TESLA_FSD=y
@@ -461,6 +462,8 @@ CONFIG_SERIAL_FSL_LPUART=y
 CONFIG_SERIAL_FSL_LPUART_CONSOLE=y
 CONFIG_SERIAL_FSL_LINFLEXUART=y
 CONFIG_SERIAL_FSL_LINFLEXUART_CONSOLE=y
+CONFIG_SERIAL_STM32=y
+CONFIG_SERIAL_STM32_CONSOLE=y
 CONFIG_SERIAL_MVEBU_UART=y
 CONFIG_SERIAL_OWL=y
 CONFIG_SERIAL_DEV_BUS=y