next-cube.c: add dummy Ethernet register to allow diagnostic to timeout

next-cube.c: add dummy Ethernet register to allow diagnostic to timeout

Add a dummy register at address 0x6000 in the MMIO memory region to allow the
initial diagnostic test to timeout rather than getting stuck in a loop
continuously writing "en_write: tx not ready" to the console.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Thomas Huth <huth@tuxfamily.org>
Message-ID: <20231220131641.592826-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>