ramips: set -march=24kec for rt305x and rt3883 to enable use of DSP extension

ramips: set -march=24kec for rt305x and rt3883 to enable use of DSP extension

All known rt305x and rt3883 implementations support at
least the 24kec instruction set which includes the DSP
extension. For rt288x, the previous value mips32r2 is
kept.

The cpu target is now set per sub-archtecture, according
to cpu-feature-overrides.h of each SoC family.

[juhosg: define ARCH_PACKAGES variable for rt305x/rt3883
in order to differentiate the packages compiled with
the 24Kec option.]

Signed-off-by: Daniel Golle <dgolle@allnet.de>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@35853 3c298f89-4303-0410-b956-a3cf2f4a3e73