[Keyboard] add support for ymd75 rev3 (#10483)

* add support for ymd75 rev3

* Update keyboards/ymd75/rev3/rules.mk

Co-authored-by: Ryan <fauxpark@gmail.com>

* Update keyboards/ymd75/rev3/rules.mk

Co-authored-by: Ryan <fauxpark@gmail.com>

* remove DESCRIPTION

* only apply offset for rev3

Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/ymd75/README.md

@@ -3,7 +3,10 @@
 75% keyboard from YMDK. YMDK sell the board and name it "YMD75", however revision 1 of the PCB has "MT84" printed on both sides.
 
 * Keyboard Maintainer: [Wayne K Jones](github.com/WarmCatUK)
-* Hardware Supported: YMD75/MT84 (rev1 and rev2) with the ATmega32a chip.
+* Hardware Supported: 
+   - rev1 (ATmega32a) 
+   - rev2 (ATmega32a) 
+   - rev3 (ATmega32u4)
 * Hardware Availability: <https://www.aliexpress.com/item/32812690592.html>
 
 Make example for this keyboard (after setting up your build environment):