ARM: v6k: select TLS register code according to V6 variants

If CONFIG_CPU_V6 is enabled, we may or may not have the TLS register.
Use the conditional code which copes with this variability.  Otherwise,
if CONFIG_CPU_32v6K is set, we know we have the TLS register on all
supported CPUs, so use it unconditionally.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>