arm: clang: Update support slightly

- Move most of the flags required into LLVM_RELFLAGS to test at build
  time instead of requiring them to be passed in.
- Update doc/README.clang to reflect this
- Switch to rpi_2 as the example as it's closer to working out of the
  box than rpi is.

Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index ce006ae..09ba027 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -16,7 +16,8 @@
 arch-$(CONFIG_CPU_PXA)		=
 arch-$(CONFIG_CPU_ARM1136)	=-march=armv5
 arch-$(CONFIG_CPU_ARM1176)	=-march=armv5t
-arch-$(CONFIG_CPU_V7)		=$(call cc-option, -march=armv7-a, -march=armv5)
+arch-$(CONFIG_CPU_V7)		=$(call cc-option, -march=armv7-a, \
+				 $(call cc-option, -march=armv7, -march=armv5))
 arch-$(CONFIG_ARM64)		=-march=armv8-a
 
 # Evaluate arch cc-option calls now