fix rock-3a spi UBOOT_TARGET_MAP

[amazingfate]

Description

In pull request #3845 I remove the UBOOT_TARGET_MAP for rkspi_loader.img to fix github workflow build error, but on the other hand spi image rkspi_loader.img will not get build in the u-boot deb package. So I add rkspi_loader.img back to UBOOT_TARGET_MAP.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• Github workflow build successfully: https://github.com/amazingfate/armbian-rock3a-images/runs/6728809237?check_suite_focus=true
• Package linux-u-boot has rkspi_loader.img.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• [] Any dependent changes have been merged and published in downstream modules