[ARM] ARMv6: add CPU_HAS_ASID configuration

Presently, we check for the minimum ARM architecture that we're
building for to determine whether we need ASID support.  This is
wrong - if we're going to support a range of CPUs which include
ARMv6 or higher, we need the ASID.

Convert the checks to use a new configuration symbol, and arrange
for ARMv6 and higher CPU entries to select it.

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