mfd: max77620: Use single-byte writes on MAX77620

The MAX77620 doesn't support bulk writes, so make sure the regmap code
breaks bulk writes into multiple single-byte writes.

Note that this is mostly cosmetic because currently only the RTC sub-
driver uses bulk writes and the RTC driver ends up using a different
regmap on the MAX77620 anyway. However, it seems like a good idea to
make this change now in order to avoid running into issues if bulk
writes are ever used by other sub-drivers sometime down the road.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>