OMAP3: control/PRCM: add omap3_ctrl_write_boot_mode()

Get rid of the open-coded scratchpad write in mach-omap2/prcm.c and
replace it with an actual API, omap3_ctrl_write_boot_mode().  While
there, get rid of the gratuitous omap_writel().

There's not much documentation available for what should wind up in
the scratchpad here, so more documentation would be appreciated.
Also, at some point, we should formalize our treatment of the scratchpad;
right now, accesses to the scratchpad are not well-documented.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Reviewed-by: Kevin Hilman <khilman@deeprootsystems.com>
Tested-by: Kevin Hilman <khilman@deeprootsystems.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>