MIPS: Refactor GIC clocksource code.

Reorganize some of the GIC clocksource driver code. Below is a list of
the various changes.

  * No longer select CSRC_GIC by default for Malta platform.
  * Limit choice for either the GIC or R4K clocksource, not both.
  * Change location in Makefile.
  * Created new 'gic_read_count' function in common 'irq-gic.c' file.
  * Change 'git_hpt_read' function in 'csrc-gic.c' to use new function.
  * Surround GIC specific code in Malta platform code with #ifdef's.
  * Only initialize the GIC clocksource if it was selected. Original
    code called it unconditionally if a GIC was found.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>