target/xilinx: Add VCU108 support

[celuk]

This PR adds vcu108 fpga port for cheshire.

Note: There are on-board switches for both vcu108 and vcu118 actually and can be used for boot mode rather than using virtual ios. I edited that part in the readme as:

@@ -40,9 +41,9 @@ Before flashing the bitstream to your device, take note of the position of onboa
 
 The reset, JTAG TAP, UART, I2C, and VGA are all connected to their onboard logic or ports. The UART has *no flow control*. The microSD slot is connected to chip select 0 of the SPI host peripheral. Serial link and GPIOs are currently not available.
 
-### Xilinx VCU118 (`vcu118`) and VCU128 (`vcu128`)
+### Xilinx VCU108 (`vcu108`), VCU118 (`vcu118`) and VCU128 (`vcu128`)
 
-Since there are no switches on these boards, the boot mode must be selected using Virtual IOs (see [Virtual IOs](#virtual_ios) below).
+Since there are no (or not enough) switches on these boards, the boot mode must be selected using Virtual IOs (see [Virtual IOs](#virtual_ios) below).

--> but still it is not a "must" and this sentence should be refactored for vcu108 and vcu118 I think. Of course, this will require USE_SWITCHES macro and xdc port definitions for them. Since it is not done for vcu118 port before, I didn't wanna touch it other than simple modification.