ARM: dts: imx: Rename "iram" node to "sram"

The device node name should reflect generic class of a device so rename
the "iram" node to "sram".  This will be also in sync with upcoming DT
schema.  No functional change.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>