sun6i: axp221: Add axp221_get_sid function

For sun6i the SID is stored in the pmic, rather then in the SoC itself,
add a function to retreive the sid.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
sunxi: Add sunxi_get_sid helper function

On sun6i the SID is stored in the pmic, rather then in the SoC itself,
add a helper function to abstract this away.

This makes our MAC address generation code also work on sun6i.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>