vendorcode/amd/agesa/fam16kb: Fix 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: Ie8817434ee0bc6c195eabe090f195512c0043ae5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/64400
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/family16kb/Kconfig

@@ -24,4 +24,7 @@ config VGA_BIOS_ID
 	  The default VGA BIOS PCI vendor/device ID should be set to the
 	  result of the map_oprom_vendev() function in northbridge.c.
 
+config AGESA_BROKEN_DATA_SECTION
+	default n
+
 endif # NORTHBRIDGE_AMD_AGESA_FAMILY16_KB