pmic: allow dump command for non contiguous register maps

Some PMICs (such as the DA9063) have non-contiguous register maps.
Attempting to read the non implemented registers returns an error
rather than a dummy value which causes 'pmic dump' to terminate
prematurely.

Fix this by allowing the PMIC driver to return -ENODATA for such
registers, which will then be displayed as '--' by pmic dump.

Use a single error code rather than any error code so that
we can distinguish between a hardware failure reading the PMIC
and a non implemented register known to the driver.

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>