dts: stm32l4: move dts.fixup to soc family level

This moves and merges the existing board-level dts.fixup files
for STM32 L4 SOC family into one soc family level dts.fixup file.
No new fixup blocks have been added, only fixup blocks, that were
part of at least one board level dts.fixup file are present in
soc family level dts.fixup file.

disco_l475_iot1 boards fixup blocks for devices connected via SPI
and I2C stay in board level dts.fixup file, because they are board
specific.

Contributes to #5707

Signed-off-by: Daniel Wagenknecht <wagenknecht.daniel@gmail.com>