rockchip: move SYS_MALLOC_SIMPLE to mach-rockchip Kconfig

Commit 1eb0c03c added
SPL_SYS_MALLOC_SIMPLE Kconfig option and changed the way it is
evaluated.

Thus, the definitions of CONFIG_SYS_MALLOC_SIMPLE in rk3***_common.h
board configs are now incorrect because CONFIG_SPL_BUILD is enabled so
CONFIG_IS_ENABLED(SYS_MALLOC_SIMPLE) will look for SPL_SYS_MALLOC_SIMPLE
instead of SYS_MALLOC_SIMPLE.

This commit fix this enabling SPL_SYS_MALLOC_SIMPLE with the new Kconfig
option by default in rockchip-mach.

Signed-off-by: Ariel D'Alessandro <ariel@vanguardiasur.com.ar>
Acked-by: Simon Glass <sjg@chromium.org>