selftests/mm: add platform independent in code comments

The in code comments for the selftest were made on the basis of 128TB
switch, an architecture feature specific to PowerPc and x86 platforms. 
Keeping in mind the support added for arm64 platforms which implements a
256TB switch, a more generic explanation has been provided.

Link: https://lkml.kernel.org/r/20230323105243.2807166-4-chaitanyas.prakash@arm.com


Signed-off-by: Chaitanya S Prakash <chaitanyas.prakash@arm.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>