Convert CONFIG_SPL_SYS_MALLOC_SIMPLE to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_SYS_MALLOC_SIMPLE

The problem here is that a few platforms have been doing:
#ifdef CONFIG_SPL_BUILD
#define CONFIG_SYS_MALLOC_SIMPLE
#endif

instead of defining CONFIG_SPL_SYS_MALLOC_SIMPLE directly.  Correct this
and update the documentation in a few places to match usage.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/clearfog_defconfig b/configs/clearfog_defconfig
index 81588c8..cbdf8f6 100644
--- a/configs/clearfog_defconfig
+++ b/configs/clearfog_defconfig
@@ -26,6 +26,7 @@
 CONFIG_DISPLAY_BOARDINFO_LATE=y
 CONFIG_SPL_MAX_SIZE=0x22fd0
 CONFIG_SPL_BSS_MAX_SIZE=0x4000
+CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_I2C=y
 CONFIG_SYS_MAXARGS=32
 CONFIG_CMD_TLV_EEPROM=y