reset: Add i.MX7 SRC reset driver

Add reset controller driver exposing various reset faculties,
implemented by System Reset Controller IP block.

Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>