Extend the SPI bus API a bit. The config structure gets an sc_cs_delay

Extend the SPI bus API a bit.  The config structure gets an sc_cs_delay
member to allow us to specify a delay between assert the CS# signal and
starting the clock.  And the transfer function gains a flags argument,
which can be used to specify a new SPI_KEEP_CS flag to keep CS# asserted
after the transfer.  This allows us to do another transfer immediately
afterwards without de-asserting CS# which is necessary for sending
commands to the upcoming Apple M1 keyboard/touchpad driver.

ok patrick@