imx: ventana: add econfig command

The Gateworks Ventana EEPROM contains a set of configuration bits that
affect the removal of device-tree nodes that support peripherals that do not
exist on sub-loaded boards. This patch adds:
 - a structure to define a config bit name, dt node alias, bit position
 - an array of supported configuration items
 - an econfig command to get/set/list configuration bits
 - use of the array when adjusting the FDT prior to boot

Signed-off-by: Tim Harvey <tharvey@gateworks.com>