arm, arm926ejs: make thumb mode compileable

in thumb mode compiler says for example for arch/arm/lib/cache-cp15.c
when enabling CONFIG_SYS_THUMB_BUILD:

{standard input}: Assembler messages:
{standard input}:373: Error: selected processor does not support Thumb mode `mrc p15,0,r4,c1,c0,0'
{standard input}:416: Error: selected processor does not support Thumb mode `mcr p15,0,r3,c2,c0,0'

so, if caches are disabled, do not use this command on arm926ejs.
used on at91 in SPL, to reduce size of SPL.

Signed-off-by: Heiko Schocher <hs@denx.de>

arch/arm/cpu/arm926ejs/cpu.c

@@ -45,7 +45,9 @@ int cleanup_before_linux (void)
 /* flush I/D-cache */
 static void cache_flush (void)
 {
+#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF))
 	unsigned long i = 0;
 
 	asm ("mcr p15, 0, %0, c7, c7, 0": :"r" (i));
+#endif
 }

arch/arm/lib/cache.c

@@ -25,10 +25,12 @@ __weak void flush_cache(unsigned long start, unsigned long size)
 #endif /* CONFIG_CPU_ARM1136 */
 
 #ifdef CONFIG_CPU_ARM926EJS
+#if !(defined(CONFIG_SYS_ICACHE_OFF) && defined(CONFIG_SYS_DCACHE_OFF))
 	/* test and clean, page 2-23 of arm926ejs manual */
 	asm("0: mrc p15, 0, r15, c7, c10, 3\n\t" "bne 0b\n" : : : "memory");
 	/* disable write buffer as well (page 2-22) */
 	asm("mcr p15, 0, %0, c7, c10, 4" : : "r" (0));
+#endif
 #endif /* CONFIG_CPU_ARM926EJS */
 	return;
 }