configs: lx2160a: Access flash memory as per spi-mem

MC_INIT and BOOT command currently access spi-nor flash memory directly.
As per spi-mem framework, flash memory access via absolute addresses is
no more possible. Use flash APIs to access memory instead of directly
using it.

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