hw/registerfields: Add shared fields macros

Occasionally a peripheral will have different operating modes, where the
MMIO layout changes, but some of the register fields have the same offsets
and behaviors.

To help support this, we add SHARED_FIELD_XX macros that create SHIFT,
LENGTH, and MASK macros for the fields that are shared across registers,
and accessors for these fields.

An example use may look as follows:
There is a peripheral with registers REG_MODE1 and REG_MODE2 at
different addreses, and both have a field FIELD1 initialized by
SHARED_FIELD().

Depending on what mode the peripheral is operating in, the user could
extract FIELD1 via
SHARED_ARRAY_FIELD_EX32(s->regs, R_REG_MODE1, FIELD1)
or
SHARED_ARRAY_FIELD_EX32(s->regs, R_REG_MODE2, FIELD1)

Signed-off-by: Joe Komlodi <komlodi@google.com>
Change-Id: Id3dc53e7d2f8741c95697cbae69a81bb699fa3cb
Message-Id: <20220331043248.2237838-2-komlodi@google.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>

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