configs: stm32mp15: increase malloc size for pre-reloc

The early malloc usage increased so 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: 280b8 / 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 0005e42..2676ff3 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=0x20000
+CONFIG_SYS_MALLOC_F_LEN=0x80000
 CONFIG_CUSTOM_SYS_INIT_SP_ADDR=0xc0100000
 CONFIG_ENV_OFFSET=0x480000
 CONFIG_ENV_SECT_SIZE=0x40000