ARM: 8042/1: iwmmxt: allow to build iWMMXt on Marvell PJ4B

Some Marvell PJ4B CPUs also implement iWMMXt extensions. With a
proper check for iWMMXt coprocessors now in place, enable it by
default on PJ4B. While at it, also allow to manually select
the corresponding Kconfig option.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>