Finding out whether a sysreg exists has little to do with that
register being accessed, so drop the is_write parameter.

Also, the reg pointer is completely unused, and we're better off
just passing the attr pointer to the function.

This result in a small cleanup of the calling site, with a new
helper converting the vGIC view of a sysreg into the canonical
one (this is purely cosmetic, as the encoding is the same).
Reviewed-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>