ARM: dts: DRA7: Fix SATA PHY node

The sata_ref_clk is a reference clock to the SATA phy.
This fixes SATA malfunction across suspend/resume or when
SATA driver is used as a module.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>