lib: sbi: Document sbi_ecall_extension members

With the introduction of the register_extensions callback the
range members (extid_start and extid_end) may now change and it
has become a bit subtle as to when a probe function should be
implemented. Document all the members and their relationship to
the register_extensions callback.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>