firmware: qcom_scm-32: replace open-coded call to __cpuc_flush_dcache_area()

Rathe rthan directly accessing architecture internal functions, provide
an "method"-centric wrapper for qcom_scm-32 to do what's necessary to
ensure that the secure monitor can see the data.  This is called
"secure_flush_area" and ensures that the specified memory area is
coherent across the secure boundary.

Acked-by: Andy Gross <agross@codeaurora.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>