OMAP3 powerdomains: remove RET from SGX power states list

The SGX device on OMAP3 does not support retention, so remove RET from the
list of possible SGX power states.  Problem debugged by Richard Woodruff
<r-woodruff2@ti.com>.

Signed-off-by: Richard Woodruff <r-woodruff2@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>