ARM: dts: keystone: preparatory patch to support K2L and K2E SOCs

Current keystone.dtsi includes SoC specific definitions for K2HK
SoCs. In order to support two addition keystone devices, k2 Edison
and K2 Lamarr and corresponding EVMs, This patch restructure the
dts files for the following:-

 - All clock nodes that are only available in k2hk SoC are moved
   from keystone-clocks.dtsi to a new k2hk-clocks.dtsi include file
 - The CPU nodes are now part of the soc specific k2hk.dtsi.
 - Change the compatibility string to ti,k2hk-evm and change
   the model name accordingly
 - Finally include k2hk-clocks.dtsi in k2hk.dtsi and that in
   k2hk-evm.dts

Cc: Olof Johansson <olof@lixom.net>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>