[hdl] add missing generic to generic_dpram in altera

This generic is dummy (does nothing), yet it is needed since the
generic component declaration in genram_pkg.vhd has such generic.
It has it, because the xilinx generic_dpram.vhd has such generic
and uses it.
TBD whether we want to attempt at providing similar functionality
for altera

modules/genrams/altera/generic_dpram.vhd

@@ -43,6 +43,7 @@ entity generic_dpram is
     g_with_byte_enable         : boolean := false;
     g_addr_conflict_resolution : string := "dont_care";
     g_init_file                : string := "none";
+    g_fail_if_file_not_found   : boolean := true; -- dummy (exists in Xilinx/generic)
     g_dual_clock               : boolean := true);
   port(
     rst_n_i : in std_logic := '1';      -- synchronous reset, active LO