ARM: dts: stm32: use the correct clock source for CEC on stm32mp151

[ Upstream commit 78ece8cce1ba0c3f3e5a7c6c1b914b3794f04c44 ]

The peripheral clock of CEC is not LSE but CEC.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>