stmhal: For SPI config, use HW_SPIx_SCK instead of HW_ENABLE_SPIx.

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.