Since commit 8f651ca6 ("pinctrl: stm32: Add get_pins_count() ops")
stm32f746-disco can't boot.

This is due to new memory allocation into STM32 pinctrl driver,
increase SYS_MALLOC_F_LEN from 0xC00 to 0xE00.
Signed-off-by: NPatrice Chotard <patrice.chotard@st.com>