ARM: keystone: dts: add a k2hk-evm specific dts file

This patch adds K2 Kepler/Hawking evm (k2hk-evm) specific dts file.
To enable re-use of bindings across multiple evms of this family,
rename current keystone.dts to keystone.dtsi and include it in the
evm specific dts file.

K2 SoC has separate ref clock inputs for various clocks. So add
separate ref clock nodes for ARM, DDR3A, DDR3B and PA PLL input
clocks in k2hk-evm.dts. While at it, rename  refclkmain to
refclksys based on device User Guide naming convention

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>