arm: Fix PT_PT table descriptor value and comment

Per ARMv7-A/ARMv8-A ARM, bits [11:2] of table descriptors are
ignored by hardware. The original comment incorrectly described
block/page descriptor fields which are not present in table
descriptors.

Use the minimal valid encoding for table descriptors by setting
PT_PT to 0x3.

This updates both arm32 and arm64 for consistency.

Signed-off-by: Gabriel Quintáns Souto <gabi.qs.mail@gmail.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Julien Grall <julien@xen.org>

xen/arch/arm/arm32/mmu/head.S

@@ -8,7 +8,7 @@
 #include <asm/page.h>
 #include <asm/early_printk.h>
 
-#define PT_PT     0xf7f /* nG=1 AF=1 SH=11 AP=01 NS=1 ATTR=111 T=1 P=1 */
+#define PT_PT     0x3   /* T=1 P=1 */
 #define PT_MEM    0xf7d /* nG=1 AF=1 SH=11 AP=01 NS=1 ATTR=111 T=0 P=1 */
 #define PT_MEM_L3 0xf7f /* nG=1 AF=1 SH=11 AP=01 NS=1 ATTR=111 T=1 P=1 */
 #define PT_DEV    0xe71 /* nG=1 AF=1 SH=10 AP=01 NS=1 ATTR=100 T=0 P=1 */

xen/arch/arm/arm64/mmu/head.S

@@ -8,7 +8,7 @@
 #include <asm/page.h>
 #include <asm/early_printk.h>
 
-#define PT_PT     0xf7f /* nG=1 AF=1 SH=11 AP=01 NS=1 ATTR=111 T=1 P=1 */
+#define PT_PT     0x3   /* T=1 P=1 */
 #define PT_MEM    0xf7d /* nG=1 AF=1 SH=11 AP=01 NS=1 ATTR=111 T=0 P=1 */
 #define PT_MEM_L3 0xf7f /* nG=1 AF=1 SH=11 AP=01 NS=1 ATTR=111 T=1 P=1 */
 #define PT_DEV    0xe71 /* nG=1 AF=1 SH=10 AP=01 NS=1 ATTR=100 T=0 P=1 */