i2c: mv64xxx: Do not use writel_relaxed()

The driver is used on PowerPC which don't provide writel_relaxed(). This
breaks the c2k and prpmc2800 default configurations. To fix the build,
turn the calls to writel_relaxed() into writel(). The impacts for ARM
should be minimal.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>