[RFC] Add support for flexcomm14 and 15

[DM777-ms]

Surface project requires flexcomm14 for spi and fc15 for i2c. Adding support for fc14 and 15 with this pr.

Addresses #230

[felipebalbi]

looks like this should be a single commit.

[jerrysxie]

Why can't we just add 14 and 15 here? If we only want to enable frg_clk support for 14 and 15, then we can add the macro for 14 and 15 + not change impl_flexcomm!().

[DM777-ms]

fc14 and fc15 have uniquely named registers that would need to be referenced by impl_flexcomm!(). Still tbd on how to update the macro to incorporate the different register sets for fc0-7, fc14, fc15.

Ex:
clkctl1.fc14fclksel().write(|w| match clk { ... });
clkctl1.frg14clksel().write(|w| match clk { ... });
clkctl1.frg14ctl.write( ... );

clkctl1.fc15fclksel().write(|w| match clk { ... });
clkctl1.frg15clksel().write(|w| match clk { ... });
clkctl1.frg15ctl.write( ... );

[felipebalbi]

I think that's something we can patch in the PACs. It looks like FC14 and FC15 should be added to the flexcomm cluster. Let me give that a try.

UPDATE: it's doable, but I didn't check to make sure all registers are exactly the same.