base-hw: flush cached zeroes in 'clear_ds(...)'

Ref #706
ahci_drv: separate PCI specifics from the rest

Ref #706
os: helper Attached_mmio

Genode::Attached_mmio is the fusion of Genode::Attached_io_mem_dataspace
and Genode::Mmio.

Ref #706
base: Mmio::wait_for(...) for whole registers

Ref #706
ahci_drv: basic support for exynos 5

This is a first version of the AHCI driver. It supports only
SATA HDDs with UDMA-133, up to 6 Gbps and native command queueing.
The more blocks one transfers with one command the higher is the
chance that the driver produces a fatal handshake error. Nevertheless
the driver is stable with one block per ATA command. Although NCQ is
used the driver doesn't queue multiple commands simultanously.

The driver was tested with a western digital HDD "WDC WD2500BEVS-08VAT1
13.01A13" (250 GB) with hw_arndale (run/ahci) and foc_arndale
(run/ahci, run/l4linux: dd). SSDs were not tested.

Fix #706