ARM: dts: dra7-evm: Enable pcf8575 (0x26 address) on i2c2 bus

This GPIO expander is used for controlling various muxes on the board.
By default select audio functionality over VIN6 by setting the P1
(vin6_sel_s0) pin to low.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>