ARM: adjust arm-smccc code for use in U-Boot

Adjust ARM SMC Calling Convention code for U-Boot:
  - Replace the license block with SPDX
  - Change path to asm-offsets.h
  - Define UNWIND() as no-op
  - Add Kconfig entry
  - Add asm-offsets

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 42f93b4..16578b8 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -174,6 +174,14 @@
 	default 64 if SYS_CACHE_SHIFT_6
 	default 32 if SYS_CACHE_SHIFT_5
 
+config ARM_SMCCC
+	bool "Support for ARM SMC Calling Convention (SMCCC)"
+	depends on CPU_V7 || ARM64
+	help
+	  Say Y here if you want to enable ARM SMC Calling Convention.
+	  This should be enabled if U-Boot needs to communicate with system
+	  firmware (for example, PSCI) according to SMCCC.
+
 config SEMIHOSTING
 	bool "support boot from semihosting"
 	help