Kconfig: Add CONFIG_SATA to enable SATA

At present CONFIG_CMD_SATA enables the 'sata' command which also brings
in SATA support. Some boards may wish to enable SATA without the command.
Add a separate CONFIG to permit this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/board/boundary/nitrogen6x/nitrogen6x.c b/board/boundary/nitrogen6x/nitrogen6x.c
index ab8b2be..1145af5 100644
--- a/board/boundary/nitrogen6x/nitrogen6x.c
+++ b/board/boundary/nitrogen6x/nitrogen6x.c
@@ -903,7 +903,7 @@
 	setup_i2c(1, CONFIG_SYS_I2C_SPEED, 0x7f, &i2c_pad_info1);
 	setup_i2c(2, CONFIG_SYS_I2C_SPEED, 0x7f, &i2c_pad_info2);
 
-#ifdef CONFIG_CMD_SATA
+#ifdef CONFIG_SATA
 	setup_sata();
 #endif