bitfield: Add non-constant field_{prep, get}() helpers

The existing FIELD_{GET,PREP}() macros are limited to compile-time
constants.  However, it is very common to prepare or extract bitfield
elements where the bitfield mask is not a compile-time constant.

To avoid this limitation, the AT91 clock driver and several other
drivers already have their own non-const field_{prep,get}() macros.
Make them available for general use by adding them to
<linux/bitfield.h>, and improve them slightly:
  1. Avoid evaluating macro parameters more than once,
  2. Replace "ffs() - 1" by "__ffs()",
  3. Support 64-bit use on 32-bit architectures,
  4. Wire field_{get,prep}() to FIELD_{GET,PREP}() when mask is
     actually constant.

This is deliberately not merged into the existing FIELD_{GET,PREP}()
macros, as people expressed the desire to keep stricter variants for
increased safety, or for performance critical paths.

Yury: use __mask within new macros.

Signed-off-by: Geert Uytterhoeven <[email protected]>
Acked-by: Alexandre Belloni <[email protected]>
Acked-by: Jonathan Cameron <[email protected]>
Acked-by: Crt Mori <[email protected]>
Acked-by: Nuno Sá <[email protected]>
Acked-by: Richard Genoud <[email protected]>
Reviewed-by: Andy Shevchenko <[email protected]>
Reviewed-by: Yury Norov (NVIDIA) <[email protected]>
Signed-off-by: Yury Norov (NVIDIA) <[email protected]>
[Linux commit: c1c6ab80b25c8db1e2ef5ae3ac8075d2c242ae13]
Signed-off-by: Mikhail Kshevetskiy <[email protected]>
Reviewed-by: David Lechner <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

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