spi: sprd: Change the hwlock support to be optional

No need to add hardware spinlock proctection due to add multiple
msater channel, so change it to be optional in documentation.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Link: https://lore.kernel.org/r/23d51f5d9c9cc647ad0c5a1fb950d3d9fb9c1303.1564125131.git.baolin.wang@linaro.org


Signed-off-by: Mark Brown <broonie@kernel.org>