spi: add DM_SPI_FLASH compatibility inline functions

spi: add DM_SPI_FLASH compatibility inline functions

To smoothly handle the transition from the legacy SPI FLASH
API to the driver model API, add the DM functions
as dummy inline functions.

Today, client code uses #if/#else conditionals, but it's better
to use if(IS_ENABLED()) to make sure all code builds fine
and avoid configuration hell, leaving the compiler remove
the dead code.

An example is cmd/sf, which could make use of those dummy
functions to drop the conditional compilation.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20241001-uboot-topic-dfu-sf-dt-v2-1-67f7acfa3ff5@linaro.org
[mkorpershoek: removed duplicate "the" from commit msg]
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>