bcm53xx: add v5.4 support

target/linux/bcm53xx/Makefile

target/linux/bcm53xx/config-5.4

target/linux/bcm53xx/image/Makefile

...-5.4/130-ARM-dts-BCM5301X-Add-DT-for-Luxul-XWC-2000.patch

...0-usb-xhci-add-support-for-performing-fake-doorbell.patch

...RM-BCM5301X-Disable-MMU-and-Dcache-during-decompres.patch

...2-ARM-dts-BCM5301X-Update-Northstar-pinctrl-binding.patch

...tches-5.4/310-ARM-BCM5301X-Add-DT-for-Netgear-R7900.patch

...M-BCM5301X-Add-power-button-for-Buffalo-WZR-1750DHP.patch

...5.4/320-ARM-dts-BCM5301X-Add-serial-to-the-bootargs.patch

...5.4/321-ARM-dts-BCM5301X-Describe-partition-formats.patch

...-UBI-Detect-EOF-mark-and-erase-all-remaining-blocks.patch

...53-add-hacky-CPU-port-fixes-for-devices-not-using-p.patch

.../linux/bcm53xx/patches-5.4/905-BCM53573-minor-hacks.patch