x86/mm/pat: Fix W=1 build kernel-doc warning

Building the kernel with W=1 generates the following warning:

  arch/x86/mm/pat/memtype.c:692: warning: Function parameter or struct member 'pfn' not described in 'pat_pfn_immune_to_uc_mtrr'

Add missing parameter documentation to fix the kernel-doc warning.

Signed-off-by: Shivank Garg <shivankg@amd.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250514062637.3287779-3-shivankg@amd.com