soc/amd/common: factor out SMU code from Picasso

The SMU mailbox access code from Picasso can be reused in the next
generation, so factor out the code to soc/amd/common/block/smu. Since
the mailbox register offsets in the indirect address space, the number
of arguments and the message IDs don't always match between different
devices, keep those in the soc-specific directories.

Change-Id: Ibaf5b91ab35428e4c771e7163c6e0c4fc50371e7
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47483
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jeremy Soller <jeremy@system76.com>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>

src/soc/amd/picasso/Kconfig

@@ -41,6 +41,7 @@ config CPU_SPECIFIC_OPTIONS
 	select SOC_AMD_COMMON_BLOCK_HDA
 	select SOC_AMD_COMMON_BLOCK_SATA
 	select SOC_AMD_COMMON_BLOCK_SMBUS
+	select SOC_AMD_COMMON_BLOCK_SMU
 	select SOC_AMD_COMMON_BLOCK_PSP_GEN2
 	select PROVIDES_ROM_SHARING
 	select BOOT_DEVICE_SUPPORTS_WRITES if BOOT_DEVICE_SPI_FLASH