staging: drm/imx: convert IPU irq driver to irq_domain_add_linear

The IPU has a lot of interrupts. Instead of allocating descs for all
of them, register a linear irq domain and create mappings as needed.
This was listed in the TODO as a prerequisite to move the IPU driver
out of staging.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>