x86/time: extend "tsc" param with "stable:socket"

x86/time: extend "tsc" param with "stable:socket"

Extend the "tsc" boot parameter is to further relax TSC restrictions and
allow it to be used on machines that guarantee reliable TSC across
sockets. This is up to board manufacturers and there's no way for the OS
to probe this property, therefore user needs to explicitly set this option.

Also make one style adjustment that is to remove the unnecessary
parenthesis around clearing TSC_RELIABLE.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>