board: freescale: p1_p2_rdb_pc: Move ifdef for USB/eLBC check to correct place

Whole section about USB/eLBC configuration seems to be P1020 specific. So
add ifdefs to not compile it on other platforms (e.g. P2020).

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>