powerpc/mm: make ioremap_bot common to all

Drop multiple definitions of ioremap_bot and make one common to
all subarches.

Only CONFIG_PPC_BOOK3E_64 had a global static init value for
ioremap_bot. Now ioremap_bot is set in early_init_mmu_global().

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/920eebfd9f36f14c79d1755847f5bf7c83703bdd.1566309262.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/nohash/32/pgtable.h

@@ -11,8 +11,6 @@
 #include <asm/mmu.h>			/* For sub-arch specific PPC_PIN_SIZE */
 #include <asm/asm-405.h>
 
-extern unsigned long ioremap_bot;
-
 #ifdef CONFIG_44x
 extern int icache_44x_need_flush;
 #endif