greybus: arche-platform: Export GPIOs after populating APBs

Populating APBs operation can potentially fail and it would be better if
we export the GPIOs towards then end of the routine, so that we don't
need to unexport them on error cases.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>