boards: arm: lpcxpresso55s69: Fix sram partitioning in devicetree
boards: arm: lpcxpresso55s69: Fix sram partitioning in devicetree

SRAM partitioning for non-secure should be done via a reserved-memory
node and not fixed-partitions.  fixed-partitions is meant for flash
style devices.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

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