RISC-V: Add dynamic ftrace support for RISC-V platforms

RISC-V: Add dynamic ftrace support for RISC-V platforms

This patch set includes the building blocks of dynamic ftrace features
for RISC-V machines.  I'm afraid I'm not very familiar with ftrace, but
the code looks OK to me.  It's been used to track down a performance
problem in our SPI driver and appears to work acceptably, but we haven't
given it a whole lot of banging yet so there might still be some bugs
lurking around somewhere.