ramips: add support for kernel 5.10

Enable testing kernel.

Delete upstreamed patches:
 0098-disable_cm.patch can be dropped, upstream fixed CM handling.

Fix compile errors by using new kernel APIs.

Fix fuzz by manually editing patches to ensure the code goes in the
right place.

For 721-NET-no-auto-carrier-off-support.patch, revert upstream commit
a307593a6 to keep the OpenWrt ralink driver operational.

Add mt7621-pci-phy patch to select REGMAP_MMIO as discussed in PR #3693
and #3952.

Rename patches to follow the 3-digit classification from the OpenWrt
Developer Guide.

Run automatic quilt refresh.

Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>

target/linux/ramips/patches-5.10/840-serial-add-ugly-custom-baud-rate-hack.patch

@@ -0,0 +1,22 @@
+From a7eb46e0ea4a11e4dfb56ab129bf816d1059a6c5 Mon Sep 17 00:00:00 2001
+From: John Crispin <blogic@openwrt.org>
+Date: Mon, 7 Dec 2015 17:31:08 +0100
+Subject: [PATCH 51/53] serial: add ugly custom baud rate hack
+
+Signed-off-by: John Crispin <blogic@openwrt.org>
+---
+ drivers/tty/serial/serial_core.c |    3 +++
+ 1 file changed, 3 insertions(+)
+
+--- a/drivers/tty/serial/serial_core.c
++++ b/drivers/tty/serial/serial_core.c
+@@ -418,6 +418,9 @@ uart_get_baud_rate(struct uart_port *por
+ 		break;
+ 	}
+ 
++	if (tty_termios_baud_rate(termios) == 2500000)
++		return 250000;
++
+ 	for (try = 0; try < 2; try++) {
+ 		baud = tty_termios_baud_rate(termios);
+