OMAP3 SDRC: Fix freeze when scaling CORE dpll to < 83Mhz

This patch fixes a bug in the CORE dpll scaling sequence which was
errouneously clearing some bits in the SDRC DLLA CTRL register and
hence causing a freeze.  The issue was observed only on platforms
which scale CORE dpll to < 83Mhz and hence program the DLL in fixed
delay mode.

Issue reported by Limei Wang <E12499@motorola.com>, with debugging
assistance from Richard Woodruff <r-woodruff2@ti.com> and Girish
Ghongdemath <girishsg@ti.com>.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Cc: Limei Wang <E12499@motorola.com>
Cc: Richard Woodruff <r-woodruff2@ti.com>
Cc: Girish Ghongdemath <girishsg@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
[paul@pwsan.com: updated patch description to include collaboration credits]