[ARM] 4712/2: Adds functions to read and write IXP4xx "feature" bits

Adds functions to read and write IXP4xx "feature" (aka "fuse")
bits, containing information about available/enabled CPU features.

The uncompress.h included by boot/compressed/misc.c resides in
a different space than rest of the kernel and thus can't use
asm/hardware.h (including asm/arch/cpu.h - which, in turn, may use
EXPORTed symbol "processor_id").

Posted to linux-arm-kernel on 2 Dec 2007 and revised.

Signed-off-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>