misc: fastrpc: Add helper function to get list and page

Add helper functions to get invoke buffer and page start pointers.

Signed-off-by: Vamsi Krishna Gattupalli <quic_vgattupa@quicinc.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220214161002.6831-10-srinivas.kandagatla@linaro.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>