mfd: sec: Split into core and transport (i2c) drivers

mfd: sec: Split into core and transport (i2c) drivers

As a preparation for adding support for Samsung's S2MPG10, which is
connected via SPEEDY / ACPM rather than I2C, split out (move) all
I2C-specific driver code into its own kernel module, sec-i2c, and
make the existing sec-core module be just the transport-agnostic core
driver kernel module.

At the same time, update all defconfigs that reference the old kconfig
symbol name.

While at it, also update file header comments and module description(s)
to drop references to 'mfd', and update comments to be C-style, not
C++.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250409-s2mpg10-v4-8-d66d5f39b6bf@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>