support for advanced SPI functions

docs/reference/pins/spi-format.md

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+# SPI Format
+
+Sets the SPI format
+
+```sig
+pins.spiFormat(8, 3);
+```
+
+### Parameters
+
+* ``bits``, 
+* ``mode``,
+
+### See also
+
+[SPI](https://developer.mbed.org/handbook/SPI)

docs/reference/pins/spi-frequency.md

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+# SPI Write
+
+Write to the SPI Slave and return the response.
+
+```sig
+pins.spiWrite(0);
+```
+
+### Parameters
+
+* ``value``: value	Data to be sent to the SPI slave
+
+### Returns
+
+* a [number](/types/number) Response from the SPI slave
+
+### See also
+
+[SPI](https://developer.mbed.org/handbook/SPI)

docs/reference/pins/spi-pins.md

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+# SPI Pins
+
+Sets the SPI MOSI, MISO and SCK pins
+
+```sig
+pins.spiPins(DigitalPin.P0, DigitalPin.P1, DigitalPin.P2);
+```
+
+### Parameters
+
+* ``MOSI``, the ``MOSI`` pin
+* ``MISO``, the ``MISO`` pin
+* ``SCK``, the ``SCK`` pin
+
+### See also
+
+[SPI](https://developer.mbed.org/handbook/SPI)