Tang mega138k

[trabucayre]

This PR contains 2 commits:

• an common.py update to adds support for Tristate (required for SDRAM phy)
• an comestic update to have a file file with same aspect of gowin generated files (IO_TYPE and misc in one line, only _p pins with pair instead of one line for _p and another one for _n).

[enjoy-digital]

Thanks @trabucayre!

[andelf]

@trabucayre @enjoy-digital This breaks Tang nano 9k, as it generates illegal code:

ERROR (EX3637) : Range is not allowed in a prefix("/data/LiteX/litex-boards/litex_boards/targets/build/sipeed_tang_nano_9k/gateware/sipeed_tang_nano_9k.v":2633)
ERROR (EX3878) : 'IO_psram_dq' is not a memory("/data/LiteX/litex-boards/litex_boards/targets/build/sipeed_tang_nano_9k/gateware/sipeed_tang_nano_9k.v":2633

IOBUF IOBUF(
	// Inputs.
	.I   (main_hyperram_tstriple0_o[0]),
	.OEN ((~main_hyperram_tstriple0_oe)),

	// Outputs.
	.O   (main_hyperram_tstriple0_i[0]),

	// InOuts.
	.IO  (IO_psram_dq[7:0][0])
);