arm64: zynqmp: Correct EG/EV part detection logic

The vcu disable bit in efuse ipdisable register is valid only
if PL powered up so, consider PL powerup status for determing
EG/EV part. If PL is not powered up, ignore EG/EV part of string.
The PL powerup status will be filled by pmufw based on PL PROGB
status in the 9th bit of version field.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>