Add functions for both reading and writing to the 8051's exram. Also,
add a little bit of documentation on how the addresses are segmented.
Signed-off-by: NConnor McAdams <conmanx360@gmail.com>
Link: https://lore.kernel.org/r/20201210160658.461739-4-conmanx360@gmail.comSigned-off-by: NTakashi Iwai <tiwai@suse.de>