board: T4240rdb: Extend cs4340_get_fw_addr() functionality

T4240RDB supports booting from 2 nor banks(default and altbank). The
corresponding defconfig can only have one entry defined and therefore,
extend cs4340_get_fw_addr() function to overwrite firmware address which
will be later used in cortina firmware.

Signed-off-by: Kuldeep Singh <kuldeep.singh@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>