soc: imx: imx8mp-blk-ctrl: add instance specific probe function

Allow the specific blk-ctrl instance to define a function, which will
be called during probe to provide device specific extensions.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-by: Lukas F. Hartmann <lukas@mntre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>