xen/arm: Update documentation in the public header arch-arm.h

xen/arm: Update documentation in the public header arch-arm.h

Minor updates to the documentation in xen/include/public/arch-arm.h:
    - Comments coding style fix
    - Typoes
    - Update the list of supported hypercalls by ARM
    - Remove unnecessary comment about 64bit.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- fixed spelling of unnecessary, corrected aarch into aarch32 ]