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

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

Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>