clk: stm32mp1: remove unnecessary CLK_DIVIDER_ALLOW_ZERO flag

The divisor of ethptp_k and ck_hse_rtc clocks is: 'value register
plus one'.
Then CLK_DIVIDER_ALLOW_ZERO flag has no effect and is useless here.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>