i2c: powermac: Prepare cleanup of powerpc's asm/prom.h

powerpc's asm/prom.h brings some headers that it doesn't
need itself.

In order to clean it up, first add missing headers in
users of asm/prom.h
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/busses/i2c-powermac.c

@@ -15,7 +15,7 @@
 #include <linux/device.h>
 #include <linux/platform_device.h>
 #include <linux/of_irq.h>
-#include <asm/prom.h>
+
 #include <asm/pmac_low_i2c.h>
 
 MODULE_AUTHOR("Benjamin Herrenschmidt <benh@kernel.crashing.org>");