The current MCHP_PFSOC_MMUART_REG_SIZE definition represent the
size occupied by all the registers. However all registers are
32-bit wide, and the MemoryRegionOps handlers are restricted to
32-bit:

  static const MemoryRegionOps mchp_pfsoc_mmuart_ops = {
      .read = mchp_pfsoc_mmuart_read,
      .write = mchp_pfsoc_mmuart_write,
      .impl = {
          .min_access_size = 4,
          .max_access_size = 4,
      },

Avoid being triskaidekaphobic, simplify by using the number of
registers.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210925133407.1259392-2-f4bug@amsat.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>