siemens: common: Make DDR_SI_TEST depend on TARGET_CAPRICORN

The DDR_SI_TEST config option is only relevant to the i.MX8 Capricorn
board.
Make DDR_SI_TEST depend on DDR_SI_TEST so that it does not show up
on other targets.

Signed-off-by: Liya Huang <1425075683@qq.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
diff --git a/board/siemens/common/Kconfig b/board/siemens/common/Kconfig
index 4ae12b1..3808257 100644
--- a/board/siemens/common/Kconfig
+++ b/board/siemens/common/Kconfig
@@ -3,4 +3,5 @@
 
 config DDR_SI_TEST
 	bool "DDR signal integrity test implementations"
+	depends on TARGET_CAPRICORN
 	default y