dt-bindings: clock: remove unused definition for stm32mp1

A copy of LTDC_PX and ETHCK_K (LTDC_K and ETHMAC_K) was introduced in
stm32mp1 dt-bindings file by mistake.
These bindings are not used and shouldn't be use to be conform with
convention name of the stm32mp1 clock IP.

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