Documentation: exynos5250-clock: Add div_i2s1 and div_i2s2

commit 79d743c1 ("clk: exynos5250: Add enum entries for divider
clock of i2s1 and i2s2") added two new clock entries. Add the clock
entry enum numbers to documentation.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>