cmd_sata: implement sata stop command

Implement sata stop command.
This introduces the __sata_stop() weak function, which mirrors
the weak __sata_initialize() function, giving users the option of
undoing the custom steps performed in overrides of sata_initialize().

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>