Convert CONFIG_SYS_NAND_DBW_8 et al to Kconfig

Convert CONFIG_SYS_NAND_DBW_8 et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_NAND_DBW_8
   CONFIG_SYS_NAND_DBW_16

Note that all instances of the code check for CONFIG_SYS_NAND_DBW_16
being defined, and then "else" to CONFIG_SYS_NAND_DBW_8 whereas all of
the configs set CONFIG_SYS_NAND_DBW_8. So we introduce
CONFIG_SYS_NAND_DBW_16 as an option.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>