DEVDISRn registers provides a mechanism for gating clocks of IP blocks
that are not used. Here we implement hwconfig option to allow users
to disable unused peripherals on the board.

For ex. If eSDHC/qDMA/eDMA are unused and with disabled status in dts,
User can enable CONFIG_FSL_DEVICE_DISABLE and set "devdis:esdhc,qdma,edma"
in hwconfig, thus ESDHC controller & eDMA/qDMA will be clock gated to
save more power.

Signed-off-by: Zhuoyu Zhang <Zhuoyu.Zhang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>