dts: nxp: Mark the USB RAM region as RAM in the MPU

Without adding a RAM entry for the USB RAM in the MPU,
USB RAM is mapped in the Peripheral Memory region
where unaligned memory accesses will cause a fault error.
Unaligned access errors were uncovered when we switch
to a different Zephyr C library where the memcpy function
implementation has unaligned accesses to the USB RAM.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>