doc: document u-boot, mmc-env-offset and u-boot, mmc-env-offset-redund

Adding documentation on the new config properties:
       'u-boot,mmc-env-offset' - overrides CONFIG_ENV_OFFSET
       'u-boot,mmc-env-offset-redundant'
                               - overrides CONFIG_ENV_OFFSET_REDUND

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>