dts: arm: nxp: rt6xx: add SRAM code region

Add SRAM code region definition to RT6xx series SOC. The RT6xx shares
SRAM partitions between the code and data bus, but a default allocation
is chosen by the SOC level devicetree. The user can modify this
allocation by changing the base address and size of the sram_code and
sram0 regions in their board devicetree.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>