dt-bindings/clock: remove STM32_SRC_CLOCK_MIN / MAX definitions

The STM32_SRC_CLOCK_MIN and STM32_SRC_CLOCK_MAX defines
are not really needed because non valid clock sources are already
filtered out by the precompiler.

Only STM32_SRC_CLOCK_MIN was used once in code and can be replaced.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>