Allow users to configure the Hz rate on implementations which do not have
a fixed clock rate.

Whenever HZ_FIXED is not set through one of its "default" settings, the
user is allowed to select a tick rate from 100, 200, 250, 300, 500Hz
and 1kHz.  This is slightly more choice than with the generic HZ
selection in kernel/Kconfig.hz (which only does 100, 250, 300Hz and
1kHz).  The reason for including 200Hz is that a greater number of other
platforms want that via the fixed rate, and 500Hz just seemed to be a
better middle value than 300Hz (which is of course very close to 250.)
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>