davinci: Support various speedgrades for MityDSP-L138 and MityARM-1808 SoMs

For the MityDSP-L138/MityARM-1808 SoMs, the speed grade can be determined
from the part number string read from the factory configuration block on
the on-board I2C PROM.  Configure the maximum CPU speed based on this
information.

This patch was tested using a MityDSP-L138 and MityARM-1808 at various
speedgrades.  Also, for code coverage, a bogus configuration was tested
as well as a configuration having an unknown part number.

Signed-off-by: Michael Williamson <michael.williamson@criticallink.com>
Tested-by: Michael Williamson <michael.williamson@criticallink.com>
Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>