x86: Fix xen serial console by removing conflicting PATA driver

x86: Fix xen serial console by removing conflicting PATA driver

The Xen serial console has been broken since the xen_domu subtarget
was merged in the generic x86 subtarget (commits 1d6879ee and 371b382a).

The reason for the broken serial console seems to be an IRQ conflict
between the serial console driver and the PATA_LEGACY driver:

[    1.330125] genirq: Flags mismatch irq 8. 00000000 (hvc_console) vs. 00000000 (platform[pata_legacy.4])
[    1.330134] hvc_open: request_irq failed with rc -16.
[    1.330148] Warning: unable to open an initial console.

Just drop the PATA_LEGACY driver from the x86/generic and x86_64
subtargets, since this driver is marked experimental and only supports
very old ISA devices anyway.  It is still included in the x86/legacy
subtarget where it rightfully belongs.

Fixes: FS#787
Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>