ata: libata: Improve LPM policies description

Improve the comment describing enum ata_lpm_policy and add comments
within that enum to describe each of the different possible values.
The enum values comments match the description given for the
CONFIG_SATA_MOBILE_LPM_POLICY config parameter.

No functional changes.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Niklas Cassel <cassel@kernel.org>