Since commit 6534d26e ("lmb: do away with arch_lmb_reserve()"),
STM32F746-disco hangs when loading device tree just before starting
kernel:

Retrieving file: /stm32f746-disco.dtb
Kernel image @ 0xc0008000 [ 0x000000 - 0x19ae00 ]
Flattened Device Tree blob at c0408000
   Booting using the fdt blob at 0xc0408000
Working FDT set to c0408000
   Loading Device Tree to c05f8000, end c05ff71c ...

Adjust STACK_SIZE to 16KB for STM32 MCUs (F4/F7 and H7) boards
to fix kernel boot process as some of these boards embeds a limited
amount of memory.

Fixes: 6534d26e ("lmb: do away with arch_lmb_reserve()")

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>