pinctrl: msm: Add ps_hold function in pinctrl-apq8064 binding documentation

This adds a function ps_hold (Power Suppy Hold Signal) in pinctrl-ap8064
documentation which was missing. This function is used to reset the targets
with apq8064 soc.

CC: "Ivan T. Ivanov" <iivanov@mm-sol.com>
CC: Stephen Boyd <sboyd@codeaurora.org>
CC: Andy Gross <agross@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Pramod Gurav <pramod.gurav@smartplayin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>