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.
- Allow configuration by a board of autorefresh number and burst length.
- Increase MPU region size to 8MiB.
- Make SDRAM region cacheable and executable.
