ufshci: enable the ufshci module to load at boot time

The ufshci driver tried to allocate a single 256KB segment for the UTP
command descriptor during boot, but failed due to memory fragmentation.
I fixed it to allocate the buffer in 8KB segments instead.

Sponsored by: Samsung Electronics
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D50933