vendorcode/amd/agesa/f15tn: Fix all improper use of .data

AGESA has a lot of code in the .data section which is for initialized
data, that in fact should be .rodata. This adds the 'CONST' keyword
everywhere it is needed.

TEST: See in the .elf file (e.g. using readelf) that there is nothing in
.data section.

Change-Id: I9593c24f764319f66a64715d91175f64edf10608
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64386
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/northbridge/amd/agesa/family15tn/Kconfig

@@ -17,4 +17,7 @@ config ECAM_MMCONF_BASE_ADDRESS
 config ECAM_MMCONF_BUS_NUMBER
 	default 64
 
+config AGESA_BROKEN_DATA_SECTION
+	default n
+
 endif # NORTHBRIDGE_AMD_AGESA_FAMILY15_TN