dpram_altera: make the instantation even more generic

When the device familly is not specified, modelsim chose the
default Stratix familly. Then, it throws up:

 Error: BYPASS value for CLOCK_ENABLE_INPUT_A is not supported in
 Stratix device family

This patch let clock_enable_xxx parameters get their default value.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>