TimeServer: Add plat_pre_init function

This is for platforms to perform any initialisation before the timer
driver is initialised.

TimeServer: Remove unnecessary plat files

Now that the ltimer will register the IRQs by itself, there's is no
longer any need to initialise the IRQs via the 'plat_post_init' function
in the platform-specific files. This commit removes the unnecessary
platform files and updates the CMakeLists file to only include the
necessary platform files.

TimeServer: Update non-plat code for new IRQ iface

With the introduction of the new IRQ interface in libsel4camkes, this
commit is a series of commits to update the components in
global-components to use the new IRQ interface. This commit in
particular is focused on the non-platform specific code in the
TimeServer component.

TimeServer: add ltimer_t back to plat_post_init

Some init functions require a reference to the ltimer.

TimeServer: Use camkes_io_ops for ltimer init

camkes_io_ops provides static implementations of io_map, io_ports and
memory allocation that is aware of camkes component connections such as
dataports.

TimeServer: move pc99 code to plat specific dir