i2c: mt65xx: add 4GB DMA mode support in i2c driver

If 4GB mode is enabled, we should add 4GB DMA mode support in i2c
driver. Set 4GB mode register to support 4GB mode.

Signed-off-by: Liguo Zhang <liguo.zhang@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
[wsa: updated commit message]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>