Add kernel 5.15 for RPI4 support

root/target/linux/bcm27xx/patches-5.15/0353-kbuild-Silence-unavoidable-dtc-overlay-warnings.patch

@@ -0,0 +1,36 @@
+From c8fbb6222aab1be712563c28e4c4031477f05693 Mon Sep 17 00:00:00 2001
+From: Phil Elwell <phil@raspberrypi.com>
+Date: Fri, 29 Jan 2021 10:34:11 +0000
+Subject: [PATCH 353/634] kbuild: Silence unavoidable dtc overlay warnings
+
+Much effort has been put into finding ways to avoid warnings from dtc
+about overlays, usually to do with the presence of #address-cells and
+size-cells, but not exclusively so. Since the issues being warned about
+are harmless, suppress the warnings to declutter the build output and
+to avoid alarming users.
+
+Signed-off-by: Phil Elwell <phil@raspberrypi.com>
+---
+ scripts/Makefile.lib | 6 ++++++
+ 1 file changed, 6 insertions(+)
+
+diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
+index d1550a5b57bd..7e3f156ac5a1 100644
+--- a/scripts/Makefile.lib
++++ b/scripts/Makefile.lib
+@@ -391,6 +391,12 @@ cmd_dtco = mkdir -p $(dir ${dtc-tmp}) ; \
+ 	$(DTC) -@ -H epapr -O dtb -o $@ -b 0 \
+ 		-i $(dir $<) $(DTC_FLAGS) \
+ 		-Wno-interrupts_property \
++		-Wno-label_is_string \
++		-Wno-reg_format \
++		-Wno-pci_device_bus_num \
++		-Wno-i2c_bus_reg \
++		-Wno-spi_bus_reg \
++		-Wno-avoid_default_addr_size \
+ 		-d $(depfile).dtc.tmp $(dtc-tmp) ; \
+ 	cat $(depfile).pre.tmp $(depfile).dtc.tmp > $(depfile)
+ 
+-- 
+2.33.1
+