spmi-pmic-arb: fix a possible null pointer dereference

If "core" memory resource is not specified, the driver could end up
dereferencing a null pointer.

Fix this by returning -EINVAL when core resource is missing.

Change-Id: Id08f7b2e109b6b2963b19dfe07f07cbfb424202b
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>