Documentation: Fix early-microcode.txt references after file rename

The file Documentation/x86/early-microcode.txt was renamed to
Documentation/x86/microcode.txt in 0e3258753f, but it was still
referenced by its old name in a three places:

* Documentation/x86/00-INDEX
* arch/x86/Kconfig
* arch/x86/kernel/cpu/microcode/amd.c

This commit updates these references accordingly.

Fixes: 0e3258753f ("x86/microcode: Document the three loading methods")
Signed-off-by: Jaak Ristioja <jaak@ristioja.ee>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/x86/00-INDEX

@@ -2,14 +2,14 @@
 	- this file
 boot.txt
 	- List of boot protocol versions
-early-microcode.txt
-	- How to load microcode from an initrd-CPIO archive early to fix CPU issues.
 earlyprintk.txt
 	- Using earlyprintk with a USB2 debug port key.
 entry_64.txt
 	- Describe (some of the) kernel entry points for x86.
 exception-tables.txt
 	- why and how Linux kernel uses exception tables on x86
+microcode.txt
+	- How to load microcode from an initrd-CPIO archive early to fix CPU issues.
 mtrr.txt
 	- how to use x86 Memory Type Range Registers to increase performance
 pat.txt