WIP: mfd: syscon: Add register stride to DT bindings.

This patch adds register stride to dt bindings so that the consumers of
the syscon could change it to there need. One of the the use case for
this feature is Qualcomm qfprom which needs a byte access to regmap
returned from syscon.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>