ARM: OMAP4: fix RFBI iclk

RFBI iclk was set to point to hacky "dss_fck", which will be removed.
Instead use "l3_div_ck", which is the proper clock for this. "l3_div_ck"
is the parent of "dss_fck", so the clock rate is the same as previously.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit.taneja@gmail.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>