hikey960.mk: fix name of hisi-sec_xloader.img

Commit f9f39124 ("hikey*: support migration to BL2_EL3 on arm-tf")
has updated the flash target to use hisi-nvme.img instead of nvme.img,
thus adopting the new naming introduced by upstream commit b5ae2c13438e
("installer: add hisi- prefix to ptable, sec_xloader, fastboot,
sec_uce_boot, and sec_usb_xloader imgs").
Unfortunately, the line dealing with sec_xloader.img was not updated.
Do it to fix "make flash".

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>