Add configuration option to skip SPI bus initialization

[doragasu]

Currently, if you configure an SPI display such as ILI9341, the driver initializes the SPI bus calling spi_bus_initialize(), and there is no way to skip this initialization. This is troublesome for boards having more devices attached to the bus, like the M5Stack boards, that share MOSI, MISO and SCLK also with the microSD card.

For these cases, it would be convenient having a configuration option to skip SPI bus initialization, and documenting that enabling this option makes the user responsible of calling spi_bus_initialize() with the appropriate values.