platform/x86/amd/hsmp: Change generic plat_dev name to hsmp_pdev

platform/x86/amd/hsmp: Change generic plat_dev name to hsmp_pdev

plat_dev is a commonly used variable name, since its made as extern now,
change it to more specific name.
Also change miscdevice hsmp_device to mdev.

Signed-off-by: Suma Hegde <suma.hegde@amd.com>
Reviewed-by: Naveen Krishna Chatradhi <naveenkrishna.chatradhi@amd.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20241021111428.2676884-7-suma.hegde@amd.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>