omap: use fncpy to copy the PM code functions to SRAM

The new fncpy API is better suited for copying some
code to SRAM at runtime. This patch changes the ad-hoc
code to the more generic fncpy API.

Tested OK on OMAP3 in low power modes (RET/OFF)
using omap2plus_defconfig with !CONFIG_THUMB2_KERNEL.
Compile tested on OMAP1/2 using omap1_defconfig.

Signed-off-by: Jean Pihet <j-pihet@ti.com>
Acked-by: Kevin Hilman <khilman@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>