Use dev_remap_addr() which supports both live device tree and flat DT
backends, avoiding direct dependency on devfdt_* helpers.
And only mapping sizeof(void *) is wrong, RNG_FIFO_COUNT_OFFSET(0x24) is
accessed in this driver. So dev_remap_addr() could also fix the mapping
size.
No functional changes.
Signed-off-by: Peng Fan <peng.fan@nxp.com>