dts: Align uart "baud-rate" property to device tree spec "current-speed"

Devicetree.org specifies that serial devices property used to set
baud rate is "current-speed", while zephyr uses "baud-rate".
Align property name in order to keep zephyr dts files compatible
with device tree specification and could be re-used from/to
Linux for instance.  We also cleanup a few SoCs that set "baud-rate" in
the SoC dts and not the board.

Jira: ZEP-2048

Change-Id: I097e7439ee46fe77c628b56531772950382fafcc
Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>