ls1021atwr: add hwconfig setting to do pin mux

Freescale LS1021ATWR share some pins. Hwconfig option is used to
allows users to choose the pin functions.

Signed-off-by: Yuan Yao <yao.yuan@freescale.com>
[York Sun: revised commit message]
Reviewed-by: York Sun <yorksun@freescale.com>