staging: slicoss: introduce register accessors that use register offsets

Introduce accessor functions that read and write registers by using a
register offset.
This is in preparation to replace the register addressing by means of the
slic_regs struct with an addressing by means of offsets.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>