clk: stm32mp1: fix bit width of hse_rtc divider

Fix the bit width of the hse rtc divider because it's off by one.

Fixes: 2c87c9d3 ("clk: stm32mp1: add RTC clock")
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>