atheros: remove 2.6.28 support

target/linux/atheros/config-2.6.28

...eros/patches-2.6.28/001-get_c0_compare_irq_function.patch

...theros/patches-2.6.28/002-mips_clocksource_init_war.patch

target/linux/atheros/patches-2.6.28/100-board.patch

target/linux/atheros/patches-2.6.28/105-ar2315_pci.patch

...et/linux/atheros/patches-2.6.28/110-ar2313_ethernet.patch

target/linux/atheros/patches-2.6.28/120-spiflash.patch

target/linux/atheros/patches-2.6.28/130-watchdog.patch

...x/atheros/patches-2.6.28/140-redboot_partition_scan.patch

...x/atheros/patches-2.6.28/200-ar2313_enable_mvswitch.patch

target/linux/atheros/patches-2.6.28/210-reset_button.patch

...x/atheros/patches-2.6.28/220-enet_micrel_workaround.patch