hw/registerfields.h: Pull FIELD etc macros out of hw/register.h

hw/register.h provides macros like FIELD which make it easy to define
shift, mask and length constants for the fields within a register.
Unfortunately register.h also includes a lot of other things, some
of which will only compile in the softmmu build.

Pull the FIELD macro and friends out into a separate header file,
so they can be used in places like target/arm files which also
get built in the user-only configs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1484937883-1068-5-git-send-email-peter.maydell@linaro.org