hikey960: Fixup the hisi-bootloader flashall script

I realized in some cases the cache partition in the hisi-bootloader
doesn't play well with the fastboot format command, so add explicit
format/size to get it to work more reliably.

Signed-off-by: John Stultz <john.stultz@linaro.org>
Change-Id: Iccb0f3a497011fd523bea4947202c434855c9b9a