stm32mp1: Move config SYS_MALLOC_LEN to Kconfig

This patch moves the the config SYS_MALLOC_LEN to
Kconfig as it is already done for zynq arch in
commit 01aa5b8f0503 ("Kconfig: Move config
SYS_MALLOC_LEN to Kconfig for zynq")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/mach-stm32mp/Kconfig b/arch/arm/mach-stm32mp/Kconfig
index 73aa382..39a2a32 100644
--- a/arch/arm/mach-stm32mp/Kconfig
+++ b/arch/arm/mach-stm32mp/Kconfig
@@ -23,6 +23,9 @@
 config SYS_SOC
 	default "stm32mp"
 
+config SYS_MALLOC_LEN
+	default 0x2000000
+
 config TARGET_STM32MP1
 	bool "Support stm32mp1xx"
 	select ARCH_SUPPORT_PSCI if !STM32MP1_TRUSTED