It is now in the DTS definition... "Devicetree"
Device tl-wr941-v4:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/ath79/dts/ar7240_tplink_tl-wr941-v4.dts;h=c8fe835e7eb22d824ad9846fc7acf1db68d001de;hb=HEAD
Included family "ar7240_tplink_tl-wr":
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/ath79/dts/ar7240_tplink_tl-wr.dtsi;h=5ceada8375622675f9b1e0b02960715da0d73a09;hb=HEAD
Included upper family "ar7240_tplink":
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/ath79/dts/ar7240_tplink.dtsi;h=de0b36bbf8ea6bd7a2955d8ff494d8daf612ea63;hb=HEAD
Included chip "ar7240":
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=target/linux/ath79/dts/ar7240.dtsi;h=730c509d7bdf625e61bf8937ab7034e6b11881ee;hb=HEAD
As you see from those, you can piggyback on the common things, and only have a small file where the device-specific differences are.
Example of recent devices:
"TP-Link TL-WR710N v2.1"
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=eb531337a779a48a2d17bc66f0d222325d6c1563
"TP-Link TL-WA801ND v3/v4"
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=9a477b833ab2aea96b9eee55acb5f9e7b01b36d8;hp=583b3e40254076693eb5227a9d9ae49eb2b0bcbf
You might read the discussions in the ar71xx-->ath79 porting guide thread: