boards: arm: l452re_p: initial support for dev board

Support for the Nucleo L452RE-P has been added. Since it's very similar
to L452RE board, both of them have been added under the same directory.
The main difference is that the P version contains a SMPS but Zephyr is
not taking advantage of it yet.

I've tested the results in a real L452RE-P board.

Signed-off-by: Xabier Crespo Álvarez <x.crespo@scrobotics.es>