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>