imx7: Update CONFIG_SPL_STACK defaults in Kconfig

Update the Kconfig entry to have the correct defaults for i.MX7
platforms, and move the existing large comment from imx7_spl.h to
doc/imx/common/imx7.txt so that it's not lost.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/Kconfig b/common/spl/Kconfig
index 173c7e0..8f3985d 100644
--- a/common/spl/Kconfig
+++ b/common/spl/Kconfig
@@ -360,7 +360,7 @@
 config SPL_SHARES_INIT_SP_ADDR
 	bool "SPL and U-Boot use the same initial stack pointer location"
 	depends on (ARM || ARCH_JZ47XX || MICROBLAZE || RISCV) && SPL_FRAMEWORK
-	default n if ARCH_SUNXI || ARCH_MX6
+	default n if ARCH_SUNXI || ARCH_MX6 || ARCH_MX7
 	default y
 	help
 	  In many cases, we can use the same initial stack pointer address for
@@ -371,6 +371,7 @@
 	hex "Initial stack pointer location"
 	depends on (ARM || ARCH_JZ47XX || MICROBLAZE || RISCV) && SPL_FRAMEWORK
 	depends on !SPL_SHARES_INIT_SP_ADDR
+	default 0x946bb8 if ARCH_MX7
 	default 0x93ffb8 if ARCH_MX6 && MX6_OCRAM_256KB
 	default 0x91ffb8 if ARCH_MX6 && !MX6_OCRAM_256KB
 	help