i2c: xlp9xx: Add support for SMBAlert

Add support for SMBus alert mechanism to i2c-xlp9xx driver.
The second interrupt is parsed to use for SMBus alert.
The first interrupt is the i2c controller main interrupt.

Signed-off-by: Kamlakant Patel <kamlakant.patel@cavium.com>
Signed-off-by: George Cherian <george.cherian@cavium.com>
Reviewed-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>