[ARM] 3912/1: Make PXA270 advertise HWCAP_IWMMXT capability

ARM patch 3756/1 added HWCAP_IWMMXT.  This patch adds support
for broadcasting that info via /proc/cpuinfo and sets it for
the CPU features of the PXA270.

I've booted 19rc3 on a pxa270 and confirmed that the /proc/cpuinfo
shows "iwmmxt" in the Features.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>