ARM: dts: keystone: Get rid of usage of skeleton.dtsi

skeleton.dtsi doesn't offer us any real benefits with most of the
parameters being overridden. So, just drop the same entirely and
introduce appropriate changes for chosen node and memory back to
our top level definition.

This also squashes the following warnings with W=1:

arch/arm/boot/dts/keystone-k2hk-evm.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2l-evm.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name
arch/arm/boot/dts/keystone-k2e-evm.dtb: Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

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