Do not attempt to flush prefetch buffer on ARM920T

[Jakub Jermář]

ARMv4 optionally supports a prefetch buffer, but ARM920T does not implement it. Yet, the initialization boot code in boot/arch/arm32/src/asm.S:

        # cp15 isb
        mcr p15, 0, r4, c7, c5, 4
        nop

and the inst_barrier() macro in kernel/arch/arm32/include/arch/barrier.h:

#if defined PROCESSOR_ARCH_armv7_a
...
#elif defined PROCESSOR_ARCH_armv6 | defined KERNEL
...
#define inst_barrier()    CP15ISB_write(0)
#else
...
#define inst_barrier()    asm volatile ("" ::: "memory")
#endif

attempt to flush it.

Note how CP15ISB_write() is used for KERNEL for <ARMv6 (and also how the barriers have possibly wrongly empty bodies in the else branch).

[Jakub Jermář]

Fix merged in mainline,2399.