i2c: ihs_i2c: Make DM compatible

Make the ihs_i2c driver DM-compatible; for legacy boards, the old functions are
retained within #ifdefs.

No board uses the new DM driver yet; this patch only lays the foundation for
future support.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/drivers/i2c/Kconfig b/drivers/i2c/Kconfig
index 17d21bb..7dbec77 100644
--- a/drivers/i2c/Kconfig
+++ b/drivers/i2c/Kconfig
@@ -268,6 +268,12 @@
 	help
 	  Define the maximum number of available I2C buses.
 
+config SYS_I2C_IHS
+        bool "gdsys IHS I2C driver"
+        depends on DM_I2C
+        help
+          Support for gdsys IHS I2C driver on FPGA bus.
+
 source "drivers/i2c/muxes/Kconfig"
 
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