MIPS: make inclusion of ROM exception vectors configurable

This adds a compile time option to include code for static
exception vectors. Static exception vectors are only needed,
when the U-Boot entry point is equal to the CPU reset exception
vector address. For instance this is the case when U-Boot is
used as ROM in Qemu or booted from parallel NOR flash. When
U-Boot is booted from RAM (e.g. loaded there by SPL), the
exception vectors need to be setup dynamically, which is done
in follow-up commits.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>