Enable the clocks during syscon probe when they are present in device tree.

This patch avoids a freeze when the SYSCFG clock is not enabled by
TF-A / OP-TEE.
Signed-off-by: NPatrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: NPatrice Chotard <patrice.chotard@foss.st.com>