ti-dm: am62xx: update firmware path

commit e0c2153d ("ti-dm: Update firmware to 08.03.00.xx")
Introduced am62xx DM "Device Manager" firmware in
folder ti-dm/am62x

Where-as
commit c2e5d59c

 ("ti-ipc: am62xx: Add IPC firmwares for am62xx")
uses other platform specific firmware in folder name "am62xx"

This naming even when comparing with other K3 generation
platform firmware is inconsistent.

This patch is to just rename the folder to stay consistent.
Also, while at it, update the WHENCE to reflect the correct location
of the dm firmware for am62xx.

Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>
Acked-by: Vishal Mahaveer <vishalm@ti.com>