Rather than using the same _MMIO() macro to define MCR registers as
singleton registers, let's use a new MCR_REG() macro to make it clear
that these registers are special and should be handled accordingly.  For
now MCR_REG() will still generate an i915_reg_t with the given offset,
but we'll change that in future patches.

Bspec: 66673, 66696, 66534, 67609
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Balasubramani Vivekanandan <balasubramani.vivekanandan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221014230239.1023689-9-matthew.d.roper@intel.com