ARM: dts: dra7xx: Fix compatible string for PCF8575 chip

The binding definition for the PCF857x GPIO expanders doesn't mention
a "ti,pcf8575" compatible string.  This is apparently because TI is
only a second source - there is no functional difference between
PCF8575 chips manufactured by TI and NXP, and the same board might be
populated with either depending on availability.

This is not a problem in practice because the I2C core uses
of_modalias_node() before matching drivers and this strips the
manufacturer name.

Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>