kernel: move CONFIG_MIPS_SEAD3 to generic

target/linux/adm5120/config-3.8

target/linux/ar71xx/config-3.8

target/linux/atheros/config-3.8

target/linux/brcm47xx/config-3.8

target/linux/brcm63xx/config-3.8

target/linux/generic/config-3.7

target/linux/generic/config-3.8

target/linux/generic/config-3.9

target/linux/lantiq/config-3.7

target/linux/lantiq/config-default

target/linux/malta/config-3.8

target/linux/ramips/rt288x/config-3.8

target/linux/ramips/rt305x/config-3.8

target/linux/ramips/rt3883/config-3.8

target/linux/rb532/config-3.8

target/linux/xburst/config-3.8