net: ipa: introduce gsi_reg_init()

Create a new source file "gsi_reg.c", and in it, introduce a new
function to encapsulate initializing GSI registers, including
looking up and I/O mapping their memory.

Create gsi_reg_exit() as the inverse of the init function.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>