ARM: OMAP2+: Add support for initializing dm814x clocks

Let's add a minimal clocks for dm814x to get it booted. This is
mostly a placeholder and relies on the PLLs being on from the
bootloader.

Note that the divider clocks work the same way as on dm816x and
am335x.

Cc: Matthijs van Duin <matthijsvanduin@gmail.com>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Tero Kristo <t-kristo@ti.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>