ARM: dts: keystone*: Use a single soc0 instance

Provide a soc0 node and reference the same to simplify dts. This also
resolves the following warnings when built with W=1:
arch/arm/boot/dts/keystone-k2hk-evm.dtb: Warning (unit_address_vs_reg): Node /soc has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2l-evm.dtb: Warning (unit_address_vs_reg): Node /soc has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2e-evm.dtb: Warning (unit_address_vs_reg): Node /soc has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2g-evm.dtb: Warning (unit_address_vs_reg): Node /soc has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2g-ice.dtb: Warning (unit_address_vs_reg): Node /soc has a reg or ranges property, but no unit name

NOTE: Though we can reformat files by reducing 1 level of indent due to
the use of soc0 phandle, we omit that change to prevent un-necessary
churn in code base.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>