gpio/tegra: convert to use linear irqdomain

The Tegra driver tries to do the work of irq_domain_add_linear()
by reserving a bunch of descriptors somewhere and keeping track
of the base offset, then calling irq_domain_add_legacy(). Let's
stop doing that and simply use the linear IRQ domain.

For this to work: use irq_create_mapping() in the IRQ iterator
so that the descriptors get allocated here.

Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Grant Likely <grant.likely@secretlab.ca>
Tested-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>