gpio: Add missing spin_lock_init in gpio-ml-ioh driver

commit 7e3a70fb7bccada029c188c89bfbf3c0a63c1500 upstream.

This bug was introduced by commit 54be5663
"gpio-ml-ioh: Support interrupt function" which adds a spinlock to struct
ioh_gpio but never init the spinlock.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>