soc: arm: lpc: move platform init implementation in the main tree

Move the implementation of z_platform_init from the NXP Hal
into the main Zephyr tree. This function is Zephyr-specific,
is part of the Zephyr module glue code, and does not need to
reside in the module repository. We also rename the function
to z_arm_platform_init. The commit does not introduce any
behavioral changes.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>