kernel: Build: Split kmod-regmap

This reduces the needed modifications to the mainline Linux kernel and
also makes the regmap package work with an out of tree kernel which
does not have these modifications.

The regmap-core is only added when it is really build as a module.
The regmap-core is normally bool so it cannot be built as a module in an
unmodified kernel. When it is selected by on other kernel module it will
always be selected as build in and it also does not show up in
$(LINUX_DIR)/modules.builtin as it is not supposed to be a kernel module.
When it is not in $(LINUX_DIR)/modules.builtin the build system expects
it to be built as a .ko file.
Just check if the module is really there and only add it in that case.

This splits the regmap package into multiple packages, one for each bus type.
This way only the bus maps which are really needed have to be added.
This also splits the I2C, SPI and MMIO regmap into separate packages to not
require all these subsystems to build them, on an unmodified upstream kernel
this also causes problems in some situations.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>

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