[codegen] Switch to FIRRTL 3.0.0 regreset

Change FIRRTL emission to use FIRRTL 3.0.0 syntax for registers with a
reset.  The old emission was:

    reg a: UInt<1>, b with : (reset => (c, d))

The new emission is:

    regreset a: UInt<1>, b, c, d

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>