davinci: am18x/da850/omap-l138: add support for higher speed grades

AM18x/DA850/OMAP-L138 SoCs have variants that can operate
at a maximum of 456 MHz at 1.3V operating point. Also the
1.2V operating point has a variant that can support a maximum
of 375 MHz.

This patch adds three new OPPs (456 MHz, 408 MHz and 372 MHz)
to the list of DA850 OPPs.

Not all silicon is qualified to run at higher speeds and
unfortunately the maximum speed the chip can support can only
be determined from the label on the package (not software
readable).

Because of this, we depend on the maximum speed grade information
to be provided to us in some board specific way. The board informs
the maximum speed grade information by setting the da850_max_speed
variable.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>