regulator: bd718x7: Only bind children when PMIC_CHILDREN is enabled

regulator: bd718x7: Only bind children when PMIC_CHILDREN is enabled

If the bd718x7 is required, but PMIC_CHILDREN is disabled, this
driver throws a compile error.  Fix this by putting the function
to bind children into an if-statement checking for PMIC_CHILDREN.

Allowing PMIC_CHILDREN to be disabled in SPL saves some space and
still permits some read/write functions to access the PMIC in
early startup.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Fabio Estevam <festevam@gmail.com>