Review SPI1 pin config

[mvertescher]

Follow up issue from #66

The following config for SPI1 is not available on every part in the F7 family:

            SCK: [
                gpio::gpiog::PG11<Alternate<AF5>>,
            ],
            MISO: [
                gpio::gpiog::PG9<Alternate<AF5>>,
            ],
            MOSI: [
                gpio::gpiod::PD7<Alternate<AF5>>,
            ],

This pin config should be disabled for F70x-F73x parts. We should double check this with the datasheets and make a minor change to spi.rs.