mailbox: qcom: Convert APCS IPC driver to use regmap

This hardware block provides more functionalities that just IPC. Convert
it to regmap to allow other child platform devices to use the same regmap.

Signed-off-by: Georgi Djakov <georgi.djakov@linaro.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>