Enable TLS register by default for ARMv7a arch

Everything that is based on ARMv7a should have a cp15 TLS register.
Enable it by default so it's not accidentally missed in newer board
configurations. In fact, this could be enabled for ARMv6 as well, but we
currently don't distinguish between ARMv5 and ARMv6 in the build system.

This can still be disabled by setting it to "false" in the board
configuration, but this shouldn't ever be needed.

Change-Id: Ic2918f32899c8bcfa482f92c98f5a192fa318470