You could try "svc #0".
mmap2 is 192 which goes in r7 (looks like there is no mmap. glibc mmap() calls mmap2).
r0 - r6 get the parameters.
r0 is replaced by the return value.
do man mmap2 for the details.
If you memmap "/dev/gpiomem" instead of "/dev/mem", you wont need the sudo.