README: Drop CONFIG_SYS_INIT_DATA_SIZE

This appears to be calculated automatically now. Drop the old reference.
Signed-off-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NBin Meng <bmeng.cn@gmail.com>
Reviewed-by: NYork Sun <york.sun@nxp.com>

README

@@ -4487,7 +4487,7 @@ Low Level (hardware related) configuration options:
 		CONFIG_SYS_GBL_DATA_OFFSET is chosen such that the initial
 		data is located at the end of the available space
 		(sometimes written as (CONFIG_SYS_INIT_RAM_SIZE -
-		CONFIG_SYS_INIT_DATA_SIZE), and the initial stack is just
+		GENERATED_GBL_DATA_SIZE), and the initial stack is just
 		below that area (growing from (CONFIG_SYS_INIT_RAM_ADDR +
 		CONFIG_SYS_GBL_DATA_OFFSET) downward.