gpio/omap: don't use linear domain mapping for OMAP1

Commit ede4d7a5 ("gpio/omap: convert gpio irq domain to linear mapping")
converted the OMAP GPIO driver to use a linear mapping for the GPIO IRQ
domain instead of using a legacy mapping. Not using a legacy mapping has
a number of benefits but it requires the platform to support SPARSE_IRQ
which currently is not supported on OMAP1.

So this change caused a regression on OMAP1 platforms [1].

Since this issue is not present on all OMAP2+ platforms, there is no need to
revert the driver to use legacy domain mapping for all the platforms.

[1]: http://www.mail-archive.com/linux-omap@vger.kernel.org/msg89005.html

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Tony Lindgren <tony@atomide.com>