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

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

In order to clean powerpc's <asm/prom.h> up in a further step,
first clean all files that include <asm/prom.h>

sungem_phy.c doesn't use any object provided by <asm/prom.h>.

But removing inclusion of <asm/prom.h> leads to the following
errors:

  CC      drivers/net/sungem_phy.o
drivers/net/sungem_phy.c: In function 'bcm5421_init':
drivers/net/sungem_phy.c:448:42: error: implicit declaration of function 'of_get_parent'; did you mean 'dget_parent'? [-Werror=implicit-function-declaration]
  448 |                 struct device_node *np = of_get_parent(phy->platform_data);
      |                                          ^~~~~~~~~~~~~
      |                                          dget_parent
drivers/net/sungem_phy.c:448:42: warning: initialization of 'struct device_node *' from 'int' makes pointer from integer without a cast [-Wint-conversion]
drivers/net/sungem_phy.c:450:35: error: implicit declaration of function 'of_get_property' [-Werror=implicit-function-declaration]
  450 |                 if (np == NULL || of_get_property(np, "no-autolowpower", NULL))
      |                                   ^~~~~~~~~~~~~~~

Remove <asm/prom.h> from included headers but add <linux/of.h> to
handle the above.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://lore.kernel.org/r/f7a7fab3ec5edf803d934fca04df22631c2b449d.1651662885.git.christophe.leroy@csgroup.euSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/sungem_phy.c

@@ -29,11 +29,7 @@
 #include <linux/mii.h>
 #include <linux/ethtool.h>
 #include <linux/delay.h>
-
-#ifdef CONFIG_PPC_PMAC
-#include <asm/prom.h>
-#endif
-
+#include <linux/of.h>
 #include <linux/sungem_phy.h>
 
 /* Link modes of the BCM5400 PHY */