armv7a: cache ttbcr and ttb0/1 on debug state entry

Instead of re-reading ttbcr and ttb0/1 whenever a virt2phys translation
is done, cache the values once when entering debug state. Use the cached
values in armv7a_mmu_translate_va().

Change-Id: I1bc5349ad2f19b2dd75bdd48468a2c1f1e028699
Signed-off-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-on: http://openocd.zylin.com/3112
Tested-by: jenkins

src/target/armv7a.h

@@ -87,6 +87,7 @@ struct armv7a_mmu_common {
 	/* following field mmu working way */
 	int32_t cached;     /* 0: not initialized, 1: initialized */
 	uint32_t ttbcr;     /* cache for ttbcr register */
+	uint32_t ttbr[2];
 	uint32_t ttbr_mask[2];
 	uint32_t ttbr_range[2];