I've tried it in this way (which is certainly wrong):
void senterp_state::main_map(address_map &map)
{
map(0x0000, 0x0014).m(m_maincpu, FUNC(hd6301y0_cpu_device::m6801_io));
map(0x4000, 0x47ff).mirror(0x3800).ram(); // internal
map(0xc000, 0xffff).rom(); // internal
}