dm: Make regmap and syscon optional

Not all boards use garbage collection in their link step, so we should avoid
adding options that rely on this for prevention of code bloat. Add separate
Kconfig options for syscon and regmap uclasses.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 5535746..e6a4590 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -52,3 +52,4 @@
 CONFIG_LED=y
 CONFIG_LED_GPIO=y
 CONFIG_SYSCON=y
+CONFIG_REGMAP=y