kernel-arch.bbclass: add arm64 support to U-Boot architecture map

to define UBOOT_ARCH, we map kernel architectures to U-Boot architectures.
In the case of arm64 kernel, we should map to arm U-boot architecture.

This patch add the exception rule to the map_uboot_arch function.

(From OE-Core rev: 0b891265716c414ade29d587fc1a3c4ea7beadbe)

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel-arch.bbclass

@@ -40,6 +40,7 @@ def map_uboot_arch(a, d):
 
     if   re.match('p(pc|owerpc)(|64)', a): return 'ppc'
     elif re.match('i.86$', a): return 'x86'
+    elif re.match('arm64$', a): return 'arm'
     return a
 
 export UBOOT_ARCH = "${@map_uboot_arch(d.getVar('ARCH', True), d)}"