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>