gianfar: Replace spin_event_timeout() with arch independent

Use arch independent code to replace the powerpc dependent
spin_event_timeout() from gfar_halt_nodisable().
Added GRS/GTS read accessors to clean-up the implementation
of gfar_halt_nodisable().

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>