GPIO: pca953x: Rework to not include commands in SPL

The command portion of the GPIO driver can only be used in full SPL so
re-work to guard the command related portions and mark it as static.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>