ARM: dts: Add missing omap4 secure clocks

The secure clocks on omap4 are similar to what we already have for dra7
in dra7_l4sec_clkctrl_regs and documented in the omap4460 TRM "Table
3-1346 L4PER_CM2 Registers Mapping Summary".

The secure clocks are part of the l4_per clock manager. As the l4_per
clock manager has now two clock domains as children, let's also update
the l4_per clockdomain node name to follow the "clock" node naming with
a domain specific compatible property.

Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh+dt@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>