It turns out that the idea of having a customizable implementation of a
regmap creation from a resource is not exactly useful. The idea was for
the new MFD-based VSC7512 driver to use something that creates a SPI
regmap from a resource. But there are problems in actually getting those
resources (it involves getting them from MFD).

To avoid all that, we'll be getting resources by name, so this custom
init_regmap() method won't be needed. Remove it.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>