am335x:Handle worst case scenario for Errata 1.0.24

In Errata 1.0.24, if the board is running at OPP50 and has a warm reset,
the boot ROM sets the frequencies for OPP100. This patch attempts to
drop the frequencies back to OPP50 as soon as possible in the SPL. Then
later the voltages and frequencies up set higher.

Cc: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Cc: Lars Poeschel <poeschel@lemonage.de>
Signed-off-by: Steve Kipisz <s-kipisz2@ti.com>
[trini: Adapt to current framework]
Signed-off-by: Tom Rini <trini@ti.com>