powerpc/40x: Map 32Mbytes of memory at startup

As reported by Carlo, 16Mbytes is not enough with modern kernels
that tend to be a bit big, so map another 16M page at boot.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/89b5f974a7fa5011206682cd092e2c905530ff46.1632755552.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/head_40x.S

@@ -27,6 +27,7 @@
 
 #include <linux/init.h>
 #include <linux/pgtable.h>
+#include <linux/sizes.h>
 #include <asm/processor.h>
 #include <asm/page.h>
 #include <asm/mmu.h>
@@ -650,7 +651,7 @@ start_here:
 	b	.		/* prevent prefetch past rfi */
 
 /* Set up the initial MMU state so we can do the first level of
- * kernel initialization.  This maps the first 16 MBytes of memory 1:1
+ * kernel initialization.  This maps the first 32 MBytes of memory 1:1
  * virtual to physical and more importantly sets the cache mode.
  */
 initial_mmu:
@@ -687,6 +688,12 @@ initial_mmu:
 	tlbwe	r4,r0,TLB_DATA		/* Load the data portion of the entry */
 	tlbwe	r3,r0,TLB_TAG		/* Load the tag portion of the entry */
 
+	li	r0,62			/* TLB slot 62 */
+	addis	r4,r4,SZ_16M@h
+	addis	r3,r3,SZ_16M@h
+	tlbwe	r4,r0,TLB_DATA		/* Load the data portion of the entry */
+	tlbwe	r3,r0,TLB_TAG		/* Load the tag portion of the entry */
+
 	isync
 
 	/* Establish the exception vector base