dev/uart: name uart_class_set DATA_SET macro UART_CLASS()

Use the macro "UART_CLASS()" for the newly created data set
'uart_class_set' as we do for other data sets.
This further hides the data set name.
Also add UART_CLASS for quicc, which was previously not done.

MFC after: 1 week
Improves: 949670f8f466 dev/uart: Use a linker set to find uart classes
Obtained from: jhb, https://github.com/freebsd/freebsd/commit/269e99ac86902127bfaee1500d8747a3c7be5912
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D43981