payloads/Kconfig: default to Skiboot payload on PPC64

PPC64 is not functional without [OPAL] with which Linux kernel
communicates to implement important functionality like a TPM driver.
In addition to providing runtime services via OPAL Skiboot also handles
some parts of the boot process (like extracting kernel from flash) and
adjusts device tree as needed.

coreboot does not currently implement OPAL and is not likely to do it
any time soon (if ever), so must use Skiboot as a second-stage firmware
to have a fully functional PPC64 device.

mb/raptor-cs/talos-2/Kconfig is updated because Skiboot doesn't fit into
512 KiB image.

[OPAL]: https://open-power.github.io/skiboot/doc/opal-spec.html

Change-Id: I49ad6094acaccf731ab6d6b45ac103d485a3179c
Signed-off-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/67070
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/Kconfig

@@ -4,7 +4,7 @@ menu "Payload"
 
 config PAYLOAD_NONE
 	bool "Don't add a payload"
-	default y if !ARCH_X86
+	default y if !ARCH_X86 && !ARCH_PPC64
 	help
 	  Select this option if you want to create an "empty" coreboot
 	  ROM image for a certain mainboard, i.e. a coreboot ROM image
@@ -17,6 +17,7 @@ if !PAYLOAD_NONE
 choice
 	prompt "Payload to add"
 	default PAYLOAD_SEABIOS if ARCH_X86
+	default PAYLOAD_SKIBOOT if ARCH_PPC64
 
 config PAYLOAD_ELF
 	bool "An ELF executable payload"

src/mainboard/raptor-cs/talos-2/Kconfig

@@ -14,7 +14,7 @@ config BOARD_SPECIFIC_OPTIONS
 	def_bool y
 	select CPU_POWER9
 	select SOC_IBM_POWER9
-	select BOARD_ROMSIZE_KB_512
+	select BOARD_ROMSIZE_KB_1024
 	select SUPERIO_ASPEED_AST2400
 	select BOOT_DEVICE_NOT_SPI_FLASH
 	select MISSING_BOARD_RESET