lasi: use constants for device register offsets

lasi: use constants for device register offsets

Instead of generating the offset based upon the physical address of the
register, add constants for each of the device registers to lasi.h and
update lasi.c to use them.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Helge Deller <deller@gmx.de>
Message-Id: <20220504092600.10048-40-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>