rockchip: back-to-bootrom: simplify the #ifdef-check for LIBCOMMON in TPL/SPL

With the finer-grained control over LIBCOMMON_SUPPORT for TPL/SPL (i.e.
with the newly introduced distinction between TPL_LIBCOMMON_SUPPORT and
SPL_LIBCOMMON_SUPPORT), we can simplify the #ifdef-check to simply use
CONFIG_IS_ENABELD.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[fixed up to use 'puts' and LIBCOMMON:]
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>