Identify the rest of the hardware constants (pret#371)

See pret/pokecrystal#972 and pret/pokecrystal#947 for details.

constants/hardware_constants.asm

@@ -119,12 +119,8 @@ DEF rBGPI             EQU $ff68 ; CGB Mode Only - Background Palette Index
 DEF rBGPD             EQU $ff69 ; CGB Mode Only - Background Palette Data
 DEF rOBPI             EQU $ff6a ; CGB Mode Only - Sprite Palette Index
 DEF rOBPD             EQU $ff6b ; CGB Mode Only - Sprite Palette Data
-DEF rUNKNOWN1         EQU $ff6c ; (FEh) Bit 0 (Read/Write) - CGB Mode Only
+DEF rOPRI             EQU $ff6c ; CGB Mode Only - Object Priority Mode
 DEF rSVBK             EQU $ff70 ; CGB Mode Only - WRAM Bank
-DEF rUNKNOWN2         EQU $ff72 ; (00h) - Bit 0-7 (Read/Write)
-DEF rUNKNOWN3         EQU $ff73 ; (00h) - Bit 0-7 (Read/Write)
-DEF rUNKNOWN4         EQU $ff74 ; (00h) - Bit 0-7 (Read/Write) - CGB Mode Only
-DEF rUNKNOWN5         EQU $ff75 ; (8Fh) - Bit 4-6 (Read/Write)
-DEF rUNKNOWN6         EQU $ff76 ; (00h) - Always 00h (Read Only)
-DEF rUNKNOWN7         EQU $ff77 ; (00h) - Always 00h (Read Only)
+DEF rPCM12            EQU $ff76 ; Channels 1 & 2 Amplitude (R)
+DEF rPCM34            EQU $ff77 ; Channels 3 & 4 Amplitude (R)
 DEF rIE               EQU $ffff ; Interrupt Enable (R/W)