Platform/ARM/Drivers: define gFdtVariableGuid in FdtPlatformDxe.dec

edk2 commit db5b28ed9e27 ("MdePkg, EmbeddedPkg: Move Fdt Table Guid to MdePkg")
removes gFdtVariableGuid while it moves the gFdtTableGuid to MdePkg.dec.

But, gFdtVariableGuid is used for development purpose to override
fdt by saving its path to load into the variable with gFdtVariableGuid.

gFdtVariableGuid is only used by FdtPlatformDxe driver and not at
any other place. So move gFdtVariableGuid into FdtPlatformDxe.dec

This patch fixes build failure of ArmJuno Platform.

Signed-off-by: Yeoreum Yun <[email protected]>

Author: LeviYeoReum

Platform/ARM/Drivers/FdtPlatformDxe/FdtPlatformDxe.dec

@@ -14,6 +14,7 @@
 
 [Guids]
   gFdtPlatformDxeTokenSpaceGuid = { 0xbfcaa0af, 0xedd4, 0x4ce7, { 0xbd, 0xb3, 0x39, 0x15, 0x07, 0x28, 0x65, 0x77 } }
+  gFdtVariableGuid = { 0x25a4fd4a, 0x9703, 0x4ba9, { 0xa1, 0x90, 0xb7, 0xc8, 0x4e, 0xfb, 0x3e, 0x57 } }
 
 [PcdsFeatureFlag.common]
   # Enable the development specific features