stm32/sdcard: Support configuring the SD/MMC bus width to 1 or 4 bits.

Some SD/MMC breakout boards don't support 4-bit bus mode.  This adds a new
macro MICROPY_HW_SDMMC_BUS_WIDTH that allows each board to define the width
of the SD/MMC bus interface used on that board, defaulting to 4 bits.