sc16is7xx: spi interface is added

spi interface for sc16is7xx is added along with Kconfig flag
to enable spi or i2c, thus in a instance we can have either
spi or i2c or both, in sync to the hw.

Signed-off-by: Rama Kiran Kumar Indrakanti <indrakanti_ram@hotmail.com>
Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>