ARM: OMAP2+: Fix ti81xx class type

Otherwise it will return true for cpu_is_omap34xx() which we don't
want for the clocks and hwmod. It's closer to am33xx for the clocks
and hwmod than to the omap34xx. We also want to be able to detect
814x and 816x separately as at least the clocks are different with
814x using a apll and 816x using a fapll for the source clocks.

Note that we can also remove omap3xxx_clk_init() call as it's wrong
and ti81xx are booting in device tree only mode.

Cc: Brian Hutchinson <b.hutchman@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>