sandbox: Enable NVMEM

This enables NVMEM for all sandbox defconfigs, enabling it to be used in
unit tests in the next few commits.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 45a6b81..b45f478 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -145,6 +145,8 @@
 CONFIG_DM_MAILBOX=y
 CONFIG_SANDBOX_MBOX=y
 CONFIG_MISC=y
+CONFIG_NVMEM=y
+CONFIG_SPL_NVMEM=y
 CONFIG_CROS_EC=y
 CONFIG_CROS_EC_I2C=y
 CONFIG_CROS_EC_LPC=y