gpio: Enable hogging support in SPL

Use the CONFIG macros to conditionally build the GPIO hogging support in
either the SPL or U-Boot, or both, depending on the configuration. Also
call the GPIO hog probe function in the common SPL board initialization
as an equivalent to adding it to the U-Boot init sequence functions.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>