broadcom/cygnus: Enable DDR auto self-refresh

Enable auto entry and auto exit self-refresh.
Configure entry idle time to 16x long count sequences.
Where a long count sequence is 1024 cycles.
The idle entry configuration is based on 32x of the DLL lock time (512 cycles).
A conservative setting to help minimize self-refresh enter/exit thrashing.

BUG=chrome-os-partner:36456
BRANCH=broadcom-firmware
TEST=When enable configuration CYGNUS_SDRAM_TEST_DDR,
print on console:

sdram initialization is completed.
test ddr start from 0x60000000 to 0x80000000
...
test ddr end: fail=0
Translation table is @ 02004000
Mapping address range [0x00000000:0x00000000) as uncached

Change-Id: Ibad220429fd52ead2933db03bec1a555f9385e53
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: 3768f82ca268fb854f8c4753916518a1efdf887d
Original-Reviewed-on: https://chrome-internal-review.googlesource.com/212125
Original-Reviewed-by: Scott Branden <sbranden@broadcom.com>
Original-Reviewed-by: Daisuke Nojiri <dnojiri@google.com>
Original-Commit-Queue: Daisuke Nojiri <dnojiri@google.com>
Original-Tested-by: Daisuke Nojiri <dnojiri@google.com>
Original-Signed-off-by: Icarus Chau <ichau@broadcom.com>
Original-Change-Id: Icac1e12745d048b32e1804a546f6b49c8b5953c0
Original-Reviewed-on: https://chromium-review.googlesource.com/265862
Original-Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Commit-Queue: Daisuke Nojiri <dnojiri@chromium.org>
Original-Trybot-Ready: Daisuke Nojiri <dnojiri@chromium.org>
Original-Tested-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-on: http://review.coreboot.org/9930
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/src/soc/broadcom/cygnus/Kconfig b/src/soc/broadcom/cygnus/Kconfig
index 821b2dc..b2f0613 100644
--- a/src/soc/broadcom/cygnus/Kconfig
+++ b/src/soc/broadcom/cygnus/Kconfig
@@ -51,6 +51,14 @@
 	bool "DDR Speed at 800MHz"
 	default y
 
+config CYGNUS_DDR_AUTO_SELF_REFRESH_ENABLE
+	bool "Enable DDR auto self-refresh"
+	default y
+	help
+	  Warning: M0 expects that auto self-refresh is enabled. Modify
+	  with caution.
+
+
 config CYGNUS_SHMOO_REUSE_DDR_32BIT
 	bool "Indicate if DDR width is 32-bit"
 	default n