Add support for SSD1331

[asmecher]

Hi! I adapted the SSD1351 driver for the SSD1331. The existing SSD1351 driver can't be used e.g. with a new init sequence (see issue #27) because the write_register function for SSD1331 requires the D/C line to be kept in command mode throughout the (potentially) multi-byte register write sequence. There's also not an "enter write mode" command for SSD1331, so D/C needs to be toggled back and forth. This could probably be cleaned up, but works for me.

[notro]

Why is write_reg8_bus8() needed?
As far as I can tell, it's the same as the default, except for removing startbyte support and pulling up 'dc' at the end.
It should not be necessary to pull up 'dc', as write_mem() is the only one that needs this, and it is taken care of.

[asmecher]

The default implementation has DC low for the first byte of a register write; see https://github.com/notro/fbtft/blob/master/fbtft-bus.c#L42 for DC level change. The SSD1331 needs DC low for the whole thing so I removed that line in the SSD1331 implementation.

[notro]

Thanks.