This adds the MPAGE (sdcc _XPAGE) SFR, which is used by sdcc in `--xstack`
mode to provide the upper byte of the address for `MOVX A, @Ri` and
`MOVX @Ri, A` instructions.
Classic 8051 uses the P2 SFR value for this, but the RTL IP block does not
seem to be using the "Port 2 module".
From the databook:
```
To replace the function of the Port 2 latch in designs that do not use a
Port 2 module, the DW8051 provides an additional special function register,
MPAGE, at SFR address 92h. During MOVX A, @Ri and MOVX @Ri, A instructions,
the DW8051 places the contents of the MPAGE register on the upper 8 address
bits (mem_addr[15:8]). This provides the paging function that is normally
provided by the Port 2 latch. The MPAGE register has no function when a
Port 2 module is used to connect external RAM.
```
Tobias Diedrich