OMAP4: PRM: add module hard reset support

Most processor modules (e.g., DSP, IVA, IPU) on OMAPs can be reset
under the control of the PRM.  This patch adds an API for this purpose
for OMAP4 devices:

int omap4_prm_is_hardreset_asserted(void __iomem *rstctrl_reg, u8 shift);
int omap4_prm_assert_hardreset(void __iomem *rstctrl_reg, u8 shift);
int omap4_prm_deassert_hardreset(void __iomem *rstctrl_reg, u8 shift);

This API is intended to be used only by the hwmod code - a subsequent
patch will add that support to hwmod.

This patch is a collaboration between Benoît Cousson <b-cousson@ti.com>
and Paul Walmsley <paul@pwsan.com>.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Benoît Cousson <b-cousson@ti.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>