fix(nxp-ddr): fix firmware buffer re-mapping issue

Firmware buffer has already been mapped when loading 1D firmware,
so the same buffer address will be re-mapped when loading 2D
firmware. Move the buffer mapping to be out of load_fw().

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>
Change-Id: Idb29d504bc482a1e7ca58bc51bec09ffe6068324