Replace CONFIG_HAS_TLS_REG with HWCAP_TLS and check for it on V6

The TLS register is only available on ARM1136 r1p0 and later.
Set HWCAP_TLS flags if hardware TLS is available and test for
it if CONFIG_CPU_32v6K is not set for V6.

Note that we set the TLS instruction in __kuser_get_tls
dynamically as suggested by Jamie Lokier <jamie@shareable.org>.

Also the __switch_to code is optimized out in most cases as
suggested by Nicolas Pitre <nico@fluxnic.net>.

Signed-off-by: Tony Lindgren <(address hidden)>
Reviewed-by: Nicolas Pitre <(address hidden)>

Change-Id: Ifca4c0792c5d86cf622e5cada77ed6acbe32943f
Refresh bcm21553 configs (obsolete TLS config)

Change-Id: I51a707a899a7bae3d34c073d25d4ff456f13136a