ata: ahci: Rename board_ahci_mobile

This board definition was originally created for mobile devices to
designate default link power managmeent policy to influence runtime
power consumption.

As this is interesting for more than just mobile designs, rename the
board to `board_ahci_low_power` to make it clear it is about default
policy.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>