spl: Ensure all SPL symbols in Kconfig have some SPL dependency

Tighten up symbol dependencies in a number of places.  Ensure that a SPL
specific option has at least a direct dependency on SPL.  In places
where it's clear that we depend on something more specific, use that
dependency instead.  This means in a very small number of places we can
drop redundant dependencies.

Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/mtd/spi/Kconfig b/drivers/mtd/spi/Kconfig
index f350c7e..f83876c 100644
--- a/drivers/mtd/spi/Kconfig
+++ b/drivers/mtd/spi/Kconfig
@@ -248,7 +248,7 @@
 
 config SPL_SPI_FLASH_MTD
 	bool "SPI flash MTD support for SPL"
-	depends on SPI_FLASH
+	depends on SPI_FLASH && SPL
 	help
           Enable the MTD support for the SPI flash layer in SPL.