ARM: keystone: Create new binding for K2G SoC

K2G SoC family is the newest version of the Keystone family of processors.

The technical reference manual for K2G can be found here:
http://www.ti.com/lit/ug/spruhy8/spruhy8.pdf

Add new bindings for K2G and the K2G evm. Also document these new bindings.

Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Nishanth Menon <[e-mail]>
Signed-off-by: Franklin S Cooper Jr <[e-mail]>
Signed-off-by: Santosh Shilimkar <[e-mail]>

Documentation/devicetree/bindings/arm/keystone/keystone.txt

···
    compatible = "ti,k2l", "ti,keystone"
 - Keystone 2 Edison
    compatible = "ti,k2e", "ti,keystone"
+- K2G
+   compatible = "ti,k2g", "ti,keystone"
 
 Boards:
 -  Keystone 2 Hawking/Kepler EVM
···
 
 -  Keystone 2 Edison EVM
    compatible = "ti,k2e-evm", "ti,k2e", "ti,keystone"
+
+-  K2G EVM
+   compatible = "ti,k2g-evm", "ti,k2g", "ti-keystone"

arch/arm/mach-keystone/keystone.c

···
 	"ti,k2hk",
 	"ti,k2e",
 	"ti,k2l",
+	"ti,k2g",
 	"ti,keystone",
 	NULL,
 };