dmaengine: dw-edma: Prepare dw_edma_probe() for builtin callers

When CONFIG_DW_EDMA=m, dw_edma_probe() is built as a module.  Previously
edma.h declared it as extern, but the implementation isn't available for
builtin callers.  A subsequent commit will add calls from
dw_pcie_host_init() and dw_pcie_ep_init(), which can only be built-in.

Make it safe for such builtin callers to call dw_edma_probe() by using
IS_REACHABLE() to define a stub when CONFIG_DW_EDMA=m.

When CONFIG_DW_EDMA=m, these builtin callers will fail to detect and
register eDMA devices, so eDMA won't be usable even if the dw-edma module
is loaded.

[bhelgaas: split to separate patch, commit log]
Link: https://lore.kernel.org/r/20230113171409.30470-25-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Vinod Koul <vkoul@kernel.org>