brcm47xx: add mips74k subtarget for newer devices

brcm47xx: add mips74k subtarget for newer devices

All new devices (with MIPS 74K CPU) are based on BCMA SoC and have
Ethernet core supported by bgmac. Create a new subtarget to generate
optimized builds.
This is just a beginning, further tweaks (like kernel configuration)
are still possible. This decreases default root.squashfs by ~128 kB.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@41025 3c298f89-4303-0410-b956-a3cf2f4a3e73