[U-Boot] ahci: add support for LBA48 data reads

[Mark Langsdorf]

The ext4 fs currently supports drive partitions greater that 2TB in size,
but the ahci driver in u-boot can't read data past the first 2TB. Add
support for lba48 reads by modifying the calls to use the lbaint_t 
type, so that lba48 block addresses can be sent to the functions, and
then add support for reading those offsets.

--Mark Langsdorf
Calxeda, Inc.