Hi,
You can generate internal reset for whatever purpose after power up or any time by using basic logic or a small counter.
signal count : unsigned(3 downto 0);
-- to prevent compiler optimising away count
attribute keep_count:boolean;
attribute keep_count of count :signal is true;
process
begin
wait until clk = '1';
if count < 15 then
count <= count + 1;
end if;
if count < 10 then
reset <= '1';
else
reset <= '0';
end if;
end process;
edit:
To be on safe side you may also add keep attribute to reset, also make sure that the (power-up don't care) is off so that the count starts at 0 always.