Use DEFINE_CLASS_0 in the arm generic timer

Rather than defining the structure manually use the DEFINE_CLASS_0
macro. As we have both an ACPI and FDT attachment we need to use the
_0 variant of the macro as DEFINE_CLASS would create two structures
with the same name.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 42353d81e3a580442a70404a4fcfcbc16c36ef3f)