ARM: mm: Add v7_flush_dcache_by_level() API.

On ARMv7 based SOC with an integrated L2 cache, there is a need
to have a flush API to operate on each cache level. In few low power
modes, L2 cache is retained where as L1 is lost. The current
v7_flush_dcache_all(), flushes all the levels and
it would be quite expensive.

So introduce v7_flush_dcache_by_level() API which takes a
parameter (cache level), and flush only on that level.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>