Kconfig: Move config SYS_MALLOC_LEN to Kconfig for zynq

This patch moves the the config SYS_MALLOC_LEN to
Kconfig. It will be just for Zynq arch and to do
will be for all other archs.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Vipul Kumar <vipul.kumar@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>