drivers/timer: stm32: Convert configuration from Kconfig to device tree

[erwango]

Use device tree to configure lptim driver with following benefits:

• Allow usage of instances other than lptim1 if needed
• Simplify driver code, specially lptim source clock selection (thanks to use of dt domain clock)

Make some related clean up in board description files.

[erwango]

CI failed due to tests/net/ failing on mps2 target:

tests/net/socket/tcp/net.socket.tcp (mps2_an385:tests/net/socket/tcp/net.socket.tcp) failed