stm32/sdram: Increase GPIO speed for SDRAM interface to "very high".

Currently all usages of mp_hal_pin_config_alt_static() set the pin speed to
"high" (50Mhz).  The SDRAM interface typically runs much faster than this
so should be set to the maximum pin speed.

This commit adds mp_hal_pin_config_alt_static_speed() which allows setting
the pin speed along with the other alternate function details.