hw/misc/grlib_ahb_apb_pnp: Fix AHB PnP 8-bit accesses

hw/misc/grlib_ahb_apb_pnp: Fix AHB PnP 8-bit accesses

The Plug & Play region of the AHB/APB bridge can be accessed
by various word size, however the implementation is clearly
restricted to 32-bit:

  static uint64_t grlib_ahb_pnp_read(void *opaque, hwaddr offset, unsigned size)
  {
      AHBPnp *ahb_pnp = GRLIB_AHB_PNP(opaque);

      return ahb_pnp->regs[offset >> 2];
  }

Similarly to commit 0fbe394a64 with the APB PnP registers,
set the MemoryRegionOps::impl min/max fields to 32-bit, so
memory.c::access_with_adjusted_size() can adjust when the
access is not 32-bit.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: KONRAD Frederic <frederic.konrad@adacore.com>
Message-Id: <20200331105048.27989-4-f4bug@amsat.org>