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 <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>