rcar: i2c: Migrate SYS_I2C_SH to Kconfig

- Migrate SYS_I2C_SH and related defines to Kconfig
- Remove currently unused SYS_I2C_SH related defines
- Cleanup related README section.

Cc: Marek Vasut <marek.vasut+renesas@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/kzm9g_defconfig b/configs/kzm9g_defconfig
index 41cd963..8e0f6af 100644
--- a/configs/kzm9g_defconfig
+++ b/configs/kzm9g_defconfig
@@ -25,6 +25,7 @@
 CONFIG_ENV_ADDR=0x40000
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SYS_I2C_LEGACY=y
+CONFIG_SYS_I2C_SH=y
 # CONFIG_MMC is not set
 CONFIG_MTD=y
 CONFIG_MTD_NOR_FLASH=y