drm/xe/xe_sysctrl: Add System Controller mailbox register definitions

drm/xe/xe_sysctrl: Add System Controller mailbox register definitions

Add register definitions for System Controller mailbox interface,
including control, data, and protocol-related fields, along with base
address and BAR configuration required by the driver.

No functional changes. This patch introduces register definitions only.

Signed-off-by: Anoop Vijay <anoop.c.vijay@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patch.msgid.link/20260327131837.2192929-11-anoop.c.vijay@intel.com