ramips: add linux 4.4 support, update mt7621 subtarget to 4.4

package/kernel/linux/modules/usb.mk

target/linux/ramips/Makefile

target/linux/ramips/dts/mt7621.dtsi

target/linux/ramips/mt7621/config-4.4

...atches-4.4/0001-arch-mips-ralink-add-mt7621-support.patch

...s/patches-4.4/0002-MIPS-ralink-add-MT7621-defconfig.patch

...patches-4.4/0004-MIPS-ralink-add-MT7621-pcie-driver.patch

...IPS-use-set_mode-to-enable-disable-the-cevt-r4k-irq.patch

...ches-4.4/0006-MIPS-ralink-add-cpu-frequency-scaling.patch

...IPS-ralink-copy-the-commandline-from-the-devicetree.patch

.../patches-4.4/0009-PCI-MIPS-adds-mt7620a-pcie-driver.patch

...s/patches-4.4/0010-arch-mips-ralink-add-spi1-clocks.patch

...s/patches-4.4/0013-owrt-hack-fix-mt7688-cache-issue.patch

...mips/patches-4.4/0014-arch-mips-cleanup-cevt-rt3352.patch

...rch-mips-do-not-select-illegal-access-driver-by-def.patch

...9-arch-mips-ralink-add-mt7621-cpu-feature-overrides.patch

...-4.4/0022-arch-mips-ralink-proper-vendor-id-srtring.patch

...ramips/patches-4.4/0024-GPIO-add-named-gpio-exports.patch

.../patches-4.4/0025-pinctrl-ralink-add-pinctrl-driver.patch

...tches-4.4/0026-DT-Add-documentation-for-gpio-ralink.patch