board: atmel: sam9x60ek: add SYS_MALLOC_F_LEN to SYS_INIT_SP_ADDR

Heap base address is computed based on SYS_INIT_SP_ADDR by
subtracting the SYS_MALLOC_F_LEN value in
board_init_f_init_reserve().

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>