spl: dm: use CONFIG_IS_ENABLED to test for the DM option

spl: dm: use CONFIG_IS_ENABLED to test for the DM option

Even though there's now a TPL_DM configuration option, the spl logic
still checks for SPL_DM and thus does not pick up the proper config
option.

This introduces the use of CONFIG_IS_ENABLED(DM) in spl.c to always
pick up the desired configuration option instead of having a
hard-coded check for the SPL variant.

Signed-off-by: Philipp Tomsich <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Tom Rini <[email protected]>