cpm2: Rework baud rate generators configuration to support external clocks.

The CPM2 BRG setup functions cpm_setbrg and cpm2_fastbrg don't support
external clocks. This patch adds a new exported __cpm2_setbrg function
that takes the clock rate and clock source as extra parameters, and moves
cpm_setbrg and cpm2_fastbrg to include/asm-powerpc/cpm2.h where they
become inline wrappers around __cpm2_setbrg.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>