api/spi: Add 2 specific control bits for special use cases.

SPI_HOLD_ON_CS can be used to ask the SPI device to keep CS on, after
the transaction. And this undefinitely, until another config is used.
This will inhibate the gpio cs delay, if any. This might be useful when
doing consecutive calls on one slave without releasing the CS.

SPI_LOCK_ON is to be used with caution as it will keep the SPI device
locked for the current config being used after each transaction. This
can be necessary if one needs to do consecutive calls on a slave without
any olher caller to interfere.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>