regmap: cache: Handle stride > 1 in sync_block_raw_flush

regcache_sync_block_raw_flush takes the address of the base register
and the address of one past the last register to write to.  "count" is
the number of registers in the range, not the number of bytes, it
should be (end addr - start addr) / stride. Without accounting for
strides greater than one, registers past the end might be synced or
the writeable_reg callback at the beginning of _regmap_raw_write will
fail and nothing will be written.

Signed-off-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Mark Brown <broonie@linaro.org>