kernel: bump 4.19 to 4.19.51

kernel: bump 4.19 to 4.19.51

Refreshed all patches.

Altered patches:
- 370-netfilter-nf_flow_table-fix-offloaded-connection-tim.patch
- 220-optimize_inlining.patch
- 640-netfilter-nf_flow_table-add-hardware-offload-support.patch

This patch also restores the initial implementation
of the ath79 perfcount IRQ issue. (78ee6b1a40b5)
It was wrongfully backported upstream initially and got reverted now.

Compile-tested on: cns3xxx, imx6
Runtime-tested on: cns3xxx, imx6

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>