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>

doc/device-tree-bindings/config.txt

@@ -21,6 +21,18 @@ u-boot,efi-partition-entries-offset
 
 	This setting will override any values configured via Kconfig.
 
+u-boot,mmc-env-offset
+u-boot,mmc-env-offset-redundant
+	If present, the values of the 'u-boot,mmc-env-offset' and/or
+	of the u-boot,mmc-env-offset-redundant' properties overrides
+	CONFIG_ENV_OFFSET and CONFIG_ENV_OFFSET_REDUND, respectively,
+	for SD/MMC devices.
+
+	Values are interpreted as the offset from the start of the
+	device, specified in bytes.  It is assumed that the setting
+	will point at the beginning of a LBA and values that are not
+	LBA-aligned will be rounded up to the next LBA address.
+
 u-boot,spl-payload-offset
 	If present (and SPL is controlled by the device-tree), this allows
 	to override the CONFIG_SYS_SPI_U_BOOT_OFFS setting using a value