Add external PLATFORM access on arm, and use it in the pl310 driver.

This allows multiple instances of SoCs that use the pl310 driver to be
built within the same kernel:

• Add access to the platform_t object from outside platform.c
• Use this with the pl310 driver

There is a new platform_pl310 interface to replace the existing code. SoCs
need to implement the init method, and if they have special requirements to
write to the two registers we care about will also need to implement the
write_ctrl and write_debug methods.

Differential Revision: https://reviews.freebsd.org/D11546