regulator: fixed: honour optionality of enable gpio

According to the binding documentation the fixed regulator enable GPIO
is optional. However so far registration thereof failed if no enable
GPIO was specified. Fix this by making it entirely optional whether an
enable GPIO is used.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>