gpio: Add driver for TI PCF8575 I2C GPIO expander

gpio: Add driver for TI PCF8575 I2C GPIO expander

TI's PCF8575 is a 16-bit I2C GPIO expander.The device features a
16-bit quasi-bidirectional I/O ports. Each quasi-bidirectional I/O can
be used as an input or output without the use of a data-direction
control signal. The I/Os should be high before being used as inputs.
Read the device documentation for more details[1].

This driver is based on pcf857x driver available in Linux v4.7 kernel.
It supports basic reading and writing of gpio pins.

[1] http://www.ti.com/lit/ds/symlink/pcf8575.pdf

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>