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

Apart from the regular AM18x/DA850/OMAP-L138 SoC operating
at 300MHz, these SoCs have variants that can operate at a
maximum of 456MHz. Variants at 408Mhz and 375 Mhz are available
as well.

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).

The EVM hardware for all these variants is the same (except
for the actual SoC populated).

U-Boot on the EVM sets up ATAG_REVISION to inform the OS
regarding the speed grade supported by the silicon. We use
this information to pass on the speed grade information to
the SoC code.

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