u-boot: Map arm64 into map for u-boot dts installation

u-boot: Map arm64 into map for u-boot dts installation

While arm64 is a valid UBOOT_ARCH (according to mkimage -A) u-boot
keeps arm64 specific dts under 'arch/arm' directory.
As the result the recipe tries to install arch/arm64 (if UBOOT_DTB
was specified) and fails with [1]. Remapping "arm64" to "arm" to fix this
issue.

[1]
| install: cannot stat '.../u-boot/1_2023.01-r0/build/arch/arm64/dts/u-boot.dtb': No such file or directory

Signed-off-by: Pavel Zhukov <pavel@zhukoff.net>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>