ARM: v6k: do not disable CPU_32v6K based on platform selection

CPU_32v6K controls whether we use the ARMv6K extension instructions in
the kernel, and in some places whether we use SMP-safe code sequences
(eg, bitops.)

MX3 prevents the selection of this option to ensure that it is not
enabled for their CPU, which is ARMv6 only.  Now that we've split the
CPU_V6 option, V6K support won't be offered for MX3 anymore.

OMAP prevents the selection of this option in an attempt to produce a
kernel which runs on architectures from ARMv6 to ARMv7 MPCore.  We now
achieve this in a different way (see the previous patches).

As such, we no longer need to offer this as a configuration option to
the user.

Tested-by: Tony Lindgren <tony@atomide.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>