Store at91_master_clock in a global variable as opposed to a compile

time constant.  This allows us to potentially change it at runtime or
autodetect it early in the boot (the latter being much more likely to
have a good outcome).

sys/arm/at91/at91.c

@@ -50,6 +50,8 @@ static struct at91_softc *at91_softc;
 
 static void at91_eoi(void *);
 
+uint32_t at91_master_clock = AT91C_MASTER_CLOCK;
+
 static int
 at91_bs_map(void *t, bus_addr_t bpa, bus_size_t size, int flags,
     bus_space_handle_t *bshp)

sys/arm/at91/at91var.h

@@ -43,4 +43,6 @@ struct at91_ivar {
 	struct resource_list resources;
 };
 
+extern uint32_t at91_master_clock;
+
 #endif /* _AT91VAR_H_ */