soc/amd/picasso: Set vboot hashing block size to 36k

On picasso's psp_verstage, the vboot hash is being calculated by
hardware using relatively expensive system calls.  By increasing the
block size, we can save roughly 150ms of boot and S3 resume time.

TEST=Build & boot see that boot time has decreased.
BRANCH=Zork
BUG=b:169217270 - Zork: SHA calculation in vboot takes too long

Signed-off-by: Martin Roth <martinroth@chromium.org>
Change-Id: I6642073357327811b415dcbcad6930ac6d2598f9
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46903
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Kangheui Won <khwon@chromium.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/picasso/Kconfig

@@ -535,6 +535,22 @@ config VBOOT_STARTS_BEFORE_BOOTBLOCK
 	  Runs verstage on the PSP.  Only available on
 	  certain Chrome OS branded parts from AMD.
 
+config VBOOT_HASH_BLOCK_SIZE
+	hex
+	default 0x9000
+	depends on VBOOT_STARTS_BEFORE_BOOTBLOCK
+	help
+	  Because the bulk of the time in psp_verstage to hash the RO cbfs is
+	  spent in the overhead of doing svc calls, increasing the hash block
+	  size significantly cuts the verstage hashing time as seen below.
+
+	  4k takes 180ms
+	  16k takes 44ms
+	  32k takes 33.7ms
+	  36k takes 32.5ms
+	  There's actually still room for an even bigger stack, but we've
+	  reached a point of diminishing returns.
+
 config CMOS_RECOVERY_BYTE
 	hex
 	default 0x51