spi: octeon: Convert driver to use readq()/writeq() functions

Remove all calls to cvmx_read_csr()/cvmx_write_csr() and use
the portable readq()/writeq() functions.

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