Hi Wasim,
Your question is how to get access to the SD card in Android, this issue has no relations to I.MX8QXP.
probably you can try this method:
1. find "frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java"
2. Open the file, and Find "grantPermissionsLPw" function.
In the function, find "if (allowedSig)" in "case PermissionInfo.PROTECTION_SIGNATURE: {"
3. Change it like bleow
if (allowedSig || perm.equals("android.permission.MOUNT_UNMOUNT_FILESYSTEMS")
|| perm.equals("android.permission.WRITE_MEDIA_STORAGE"))
Then try it.
If the way can't solve your problem, you can search related solution from internet, or android development site.
Hope above answer is helpful to you.
Have a nice day!
B.R,
Weidong