ARM: mm: Fix ECC mem policy printk

ARM: mm: Fix ECC mem policy printk

ECC policy can be applied to the whole system
when this bit is implemented by SoC vendor
(IMP - bit 9 - in L1 page table entry format).
When this bit is not implemented by SoC vendor
it doesn't mean that system has no other way
how to do ECC.
This patch ensures to show this message only when ECC
is requested via cmd line ecc=on and runs on
appropriate ARM core.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>