i2c: xlr: add interrupt support for Sigma Designs chips

The Sigma Designs variant of this controller has the ability to generate
interrupts.  This is controlled using two additional registers, oddly
enough overlapping with the defined but unused HDSTATIM.

This patch adds support for using this feature instead of busy-looping
if an IRQ is specified.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>