sandbox: enable clk command on the sandbox

Enabling the clk command on the sandbox will allow us to write tests
for it.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/configs/sandbox_flattree_defconfig b/configs/sandbox_flattree_defconfig
index dd62adb..0313fa0 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -44,6 +44,7 @@
 CONFIG_CMD_MEMINFO=y
 CONFIG_CMD_MX_CYCLIC=y
 CONFIG_CMD_MEMTEST=y
+CONFIG_CMD_CLK=y
 CONFIG_CMD_DEMO=y
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_GPT=y