mfd: rsmu: Split core code into separate module

Linking a file into two modules can have unintended side-effects
and produces a W=1 warning:

scripts/Makefile.build:236: drivers/mfd/Makefile: rsmu_core.o is added to multiple modules: rsmu-i2c rsmu-spi

Make this one a separate module instead.

Fixes: a1867f85e06e ("mfd: Add Renesas Synchronization Management Unit (SMU) support")
Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/mfd/Makefile

···
 obj-$(CONFIG_MFD_ATC260X)	+= atc260x-core.o
 obj-$(CONFIG_MFD_ATC260X_I2C)	+= atc260x-i2c.o
 
-rsmu-i2c-objs			:= rsmu_core.o rsmu_i2c.o
-rsmu-spi-objs			:= rsmu_core.o rsmu_spi.o
-obj-$(CONFIG_MFD_RSMU_I2C)	+= rsmu-i2c.o
-obj-$(CONFIG_MFD_RSMU_SPI)	+= rsmu-spi.o
+obj-$(CONFIG_MFD_RSMU_I2C)	+= rsmu_i2c.o rsmu_core.o
+obj-$(CONFIG_MFD_RSMU_SPI)	+= rsmu_spi.o rsmu_core.o

drivers/mfd/rsmu_core.c

···
 
 	return ret;
 }
+EXPORT_SYMBOL_GPL(rsmu_core_init);
 
 void rsmu_core_exit(struct rsmu_ddata *rsmu)
 {
 	mutex_destroy(&rsmu->lock);
 }
+EXPORT_SYMBOL_GPL(rsmu_core_exit);
 
 MODULE_DESCRIPTION("Renesas SMU core driver");
 MODULE_LICENSE("GPL");