bitops.h: Add functions to extract and deposit bitfields

bitops.h: Add functions to extract and deposit bitfields

Add functions deposit32(), deposit64(), extract32() and extract64()
to extract and deposit bitfields in 32 and 64 bit words. Based on
ideas by Jia Liu and Avi Kivity.

Suggested-by: Jia Liu <proljc@gmail.com>
Suggested-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>