davinci: da830 EVM: make machine name string consistent with da850

The machine name string shows up in /proc/cpuinfo under 'Hardware' and
can be used by userspace apps.  Make the format consistent with the
DA850/OMAP-l138 EVM by adding the '-' between OMAP and L137.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>