Thanks Maldon.
I applied the dsdt.aml you sent. Now intel graphics not working.
If onboard graphics enabled in bios, boot will get stuck at " Couldn't allocate runtime area "..
If onboard graphics disabled in bios, system boots up fine with AMD RX 580.
Sending "sendme" file.
Send me JIBINs-iMac.zip
I have shutdown issue. Sending the send_me.zip file below :
https://docs.google.com/open?id=1KwODWDE6xNln0dGiCa1Ry-FyP9drPuQq
Pls send me a patched dsdt.