ppc/pnv: Implement ADU access to LPC space

ppc/pnv: Implement ADU access to LPC space

One of the functions of the ADU is indirect memory access engines that
send and receive data via ADU registers.

This implements the ADU LPC memory access functionality sufficiently
for IBM proprietary firmware to access the UART and print characters
to the serial port as it does on real hardware.

This requires a linkage between adu and lpc, which allows adu to
perform memory access in the lpc space.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>