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

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.