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>
12 files changed