defconfig: msm-copper: Enable SPARSE_IRQ

SPARSE_IRQ is used to support our future PMIC designs, which have
up to 32768 theoretical interrupts. Without this feature,
supporting this device would cost huge amounts of memory.

Enabling this feature has a small performance hit since
irq_to_desc() is now implemented with a radix tree lookup.
However, the measured cost of this lookup is small compared to
the functional benefit we gain from this.

Change-Id: I19782fbe9f995c1770b78fb8d22430ac5cebf8b4
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>