ARM: davinci: select SPARSE_IRQ

Everything is in place now for SPARSE_IRQ. Select it and set
DAVINCI_INTC_START to NR_IRQS.

We now need to include mach/irqs.h in a couple places as it is no
longer indirectly included after selecting SPARSE_IRQ.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: David Lechner <david@lechnology.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>