ahci-platform: "Library-ise" ahci_probe functionality

ahci_probe consists of 3 steps:
1) Get resources (get mmio, clks, regulator)
2) Enable resources, handled by ahci_platform_enable_resouces
3) The more or less standard ahci-host controller init sequence

This commit refactors step 1 and 3 into separate functions, so the platform
drivers for AHCI implementations which need a specific order in step 2,
and / or need to do some custom register poking at some time, can re-use
ahci-platform.c code without needing to copy and paste it.

Note that ahci_platform_init_host's prototype takes the 3 non function
members of ahci_platform_data as arguments, the idea is that drivers using
the new exported utility functions will not use ahci_platform_data at all,
and hopefully in the future ahci_platform_data can go away entirely.

tj: Minor comment formatting updates.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>