drivers: ssd16xx: Add an API to read raw RAM contents

Add the ssd16xx_read_ram() function that can be used to read the raw
contents of the two display RAM in the controller. This function is
similar to display_read(), but lets the caller specify the RAM to
operate on. The intention is that this can be used to read out the
contents of the old frame buffer after a partial refresh that
automatically swaps the black/red buffers.

Signed-off-by: Andreas Sandberg <andreas@sandberg.uk>