make it possible to enable/disable UART drivers

make it possible to enable/disable UART drivers

All the UART drivers are silent options. Add one line descriptions so
that can be de/selected via menuconfig.

Add an x86 dependency to HAS_EHCI: EHCI PCI has not been used on ARM. In
fact, it depends on PCI, and moreover we have drivers for several
embedded UARTs for various ARM boards.

NS16550 remains not selectable on x86.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>
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Changes in v4:
- improve commit message
- remove prompt for HAS_EHCI

Changes in v3:
- NS16550 prompt if ARM

Changes in v2:
- make HAS_EHCI depend on x86