tx2: Remove clock driver

Since the tx2 clock driver is heavily reliant on the libtx2bpmp library
in project_libs, we remove the tx2 clock driver from this repository and
place it into libplatsupportports in the project_libs library.

tx2: Add tx2 clock driver

This clock driver is essentially an shim between the clock_sys_t
interface and the BPMP clock ABI.

platsupport: Add initial TX2 support

The TX2 uses mostly the same default serial and timer devices. The
differences are:
- Extra interrupt routing for timers
- Different MMIO and interrupt mappings compared to TX1 and TK1