Kconfig: remove a few puzzling comments

These comments mention CONFIG options that do not exist: not as a symbol
in a Kconfig file (without the CONFIG_ prefix) and neither as a symbol
(with that prefix) in the code.

There's one reference to XSCALE_PMU_TIMER as a negative dependency.
But XSCALE_PMU_TIMER is never defined (CONFIG_XSCALE_PMU_TIMER is
also unused in the code). It shows up with type "unknown" if you search
for it in menuconfig. Apparently a negative dependency on an unknown
symbol is always true. That negative dependency can be removed too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>