Architecturally the cache contents are undefined so it might well
contain stale data at reset. So better be save than sorry.

I verifyed that the added instructions are defined for both, ARMv6 and
ARMv7, using the ARM Architecture Reference Manual, ARMv7-A and ARMv7-R
edition (ARM DDI 0406C.c). For the already existing mcr instruction see
the newly added comment.

This patch also unifies handling of ARMv6 and ARMv7, the isb instruction
can also be done on the latter via mcr which simplifies the code a bit.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>