configs: stm32mp15: increase malloc size for pre-reloc

With support of SCMI in OP-TEE, the early malloc usage
increase, the associated defined CONFIG_SYS_MALLOC_F_LEN
need to be increased.

For example, for stm32mp15_defconfig and
stm32mp157c-dk2-scmi.dtsi, we have:

Early malloc usage: 14098 / 80000

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
diff --git a/configs/stm32mp15_defconfig b/configs/stm32mp15_defconfig
index 3452403..e5a2996 100644
--- a/configs/stm32mp15_defconfig
+++ b/configs/stm32mp15_defconfig
@@ -1,7 +1,7 @@
 CONFIG_ARM=y
 CONFIG_ARCH_STM32MP=y
 CONFIG_TFABOOT=y
-CONFIG_SYS_MALLOC_F_LEN=0x3000
+CONFIG_SYS_MALLOC_F_LEN=0x20000
 CONFIG_ENV_OFFSET=0x480000
 CONFIG_ENV_SECT_SIZE=0x40000
 CONFIG_DEFAULT_DEVICE_TREE="stm32mp157c-ev1"