cpufreq: omap: remove omap-cpufreq

We can now use cpufreq-cpu0 driver using DT entries.
remove the redundant omap-cpufreq driver from the tree.
Remove MAINTAINERS file entry for the same as well.

Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Nishanth Menon <nm@ti.com>