mimxrt/machine_spi: Add the SPI class to the machine module.

This class supports SPI bus controller mode, with blocking transfers.

SPI device numbers start at 0, to comply with the pinout of the Teensy
boards.  With the configured clock frequency the fastest baud rate is
33MHz.  For messages longer 16 bytes DMA is used.  The class uses the
existing framework with extmod/machine_spi.c.

Extended driver options:

- drive=n with n being between 1 and 6 or PIN.POWER_1 to PIN.POWER_6.
  Since the pins used by the SPI are fixed, no Pin settings can be made.
  Thus the drive option is added allowing to control ringing and crosstalk
  on the connection.

- gap_ns=nnnnn is the time between sent data items in a frame given in ns.
  Default is 2 clock cycles.

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