stm32f7: increase the max no of pin configuration to 70

The number of pins to be configured could be more than 50 e.g. in case
of sdram controller, there are about 56 pins (32 data lines, 12 address
& some control signals).

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
cc: Christophe KERELLO <christophe.kerello@st.com>