Such constants are MCU specific so shouldn't be specified in the board
config file (else it leads to too much duplication of code).
This patch also adds I2C timing values for the F767/F769 for 100k, 400k
and 1MHz I2C bus frequencies.
There is a minor functional change with this patch, that the GPIO are now
configured in fast mode, whereas they were in high speed mode before. But
the SDIO should still work because SD CK frequency is at most 25MHz.
This follows the pattern of other peripherals (I2C, SPI) to specify the
pins using pin objects instead of a pair of GPIO port and pin number. It
makes it easier to customise the UART pins for a particular board.
Previously, SPI was configured by a board defining MICROPY_HW_ENABLE_SPIx
to 0 or 1. Now, the board should define MICROPY_HW_SPIx_SCK, MISO, MOSI
and NSS. This makes it the same as how I2C is configured.
USB serial is now working for F7.
Internal file storage is now working for F7. The flash is laid out a bit
differently to the F4 - 4 x 32K, 1 x 128K with the rest 256K, so the
internal storage is 96K.
Added more pind definitions for STM32F7DISC board. Made USART1 be the
default HWUART repl. The STLINK usb connector also looks like a USB
serial port which is attached to USART1 on the STM32F7DISC.
