Try to work with linux kernel specified compat strings

[tmagik]

Work with the new linux 'compatible' strings for uart

[jrtc27]

It's currently compatible = "sifive,fu540-c000-uart", "sifive,uart0";, so after #202 it should now pick up the sifive,uart0 in there again. I don't see a reason to match on the more-specific one given the driver doesn't currently do anything that doesn't apply to the generic SiFive UART; the whole point of having compatible be a list is so that generic drivers only need to match the generic thing, with the more-specific entry matched only when necessary.