Convert CONFIG_CMD_PCA953X to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_PCA953X

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/configs/snapper9g20_defconfig b/configs/snapper9g20_defconfig
index f524a66..e2b5841 100644
--- a/configs/snapper9g20_defconfig
+++ b/configs/snapper9g20_defconfig
@@ -23,6 +23,7 @@
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_FAT=y
+CONFIG_CMD_PCA953X=y
 # CONFIG_MMC is not set
 CONFIG_USB=y
 CONFIG_USB_STORAGE=y