spi: octeon: Put register offsets into a struct

Instead of hard-coding the register offsets put them into a struct
and set them in the probe function.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Tested-by: Steven J. Hill <steven.hill@cavium.com>
Signed-off-by: Mark Brown <broonie@kernel.org>