This patch restructures the mips_r4k.c code. Goal is to make
it easier to add additional machines in the future. Taking
ARM as example, two additional files mips_timer.c and
mips_pic.c are introduced. The corresponding code from
mips_r4k.c is moved to these files. With this, code reuse is
possible for other machines as well and unnecessary code
duplication can be avoided.

Additionally, machine is renamed from mips_machine to
mips_r4k_machine. Again, this makes it easier to add
addional mips_xxx_machines in future.

This patch has no functional changes. Only exitsting code is
moved, machine name changes and empty function
cpu_mips_irqctrl_init() is removed.

Signed-off-by: Dirk Behme <dirk.behme_at_gmail.com>