i2c: qup: Add Device tree support for Qualcomm I2C controller

Device tree support is added for Qualcomm I2C controller and
documentation is provided for required and optional device-node
properties.
Slave devices can be added using register_board_info (boards file),
or by listing them as child of I2C of_node in the device tree.

Change-Id: I10e9b8c7805c49dfe6a51975129f1599bbca84eb
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>