ahci: move ahci_sb600_softreset to libahci.c and rename it

ahci_sb600_softreset was in ahci.c. This function is used
to fix soft reset failure and renames as ahci_pmp_retry_softreset
in libahci.c.

Signed-off-by: Yuan-Hsin Chen <yhchen@faraday-tech.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>