platform/x86/amd/hsmp: create plat specific struct

platform/x86/amd/hsmp: create plat specific struct

Having a separate platform device structure helps in future, to
contain platform specific variables and other data.

Also, define macros for dev nodes

Signed-off-by: Suma Hegde <suma.hegde@amd.com>
Reviewed-by: Naveen Krishna Chatradhi <nchatrad@amd.com>
Link: https://lore.kernel.org/r/20231010120310.3464066-1-suma.hegde@amd.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>