MIPS: FW: Remove obsolete header file for MTI platforms.

Remove 'arch/mips/include/asm/mips-boards/prom.h' and get rid of
all inclusions of it by Malta and SEAD-3 platforms.

[ralf@linux-mips.org: Fold in John Crispin <blogic@openwrt.org>'s "MIPS:
ar7 powertv build"].

[ralf@linux-mips.org: Fold in John Crispin <blogic@openwrt.org>'s "MIPS:
unbreak powertv build"].

[ralf@linux-mips.org: Test. Build. Your. Fscking. Code. Or...]

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