spl: configure 'return to bootrom' separately for SPL and TPL

On the RK3368, we want our TPL to use the 'return to bootrom' boot
method (to have the bootrom load up the SPL stage) and then continue
with different boot methods (MMC, SPI, etc.) from SPL.

This adds the config option needed to control the availabily of the
'return to bootrom' boot-method separately for the TPL stage.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

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