xen/arm: gic: Remove pointless assertion against enum gic_sgi

xen/arm: gic: Remove pointless assertion against enum gic_sgi

The Arm Compiler will complain that the assertions ASSERT(sgi < 16) are
always true. This is because sgi is an item of the enum gic_sgi and
should always contain less than 16 SGIs.

Rather than using ASSERTs, introduce a new item in the enum that could
be checked against a build time.

Take the opportunity to remove the specific assigned values for each
item. This is fine because enum always starts at zero and values will be
assigned by increment of one. None of our code also rely on hardcoded
value.

[stefano: grammar fixes in commit message]

Signed-off-by: Julien Grall <julien@xen.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
CC: Andrii Anisov <andrii_anisov@epam.com>