base: arch_topology: update Kconfig help description

Commit 5d777b18 ("arch_topology: Make cpu_capacity sysfs node as read-only")
made cpu_capacity sysfs node read-only. Update the GENERIC_ARCH_TOPOLOGY
Kconfig help section to reflect the same.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>