ARM: OMAP2+: Configure voltage controller for cpcap to low-speed

Looks like the i2c timings in high-speed mode do not work properly to
allow us to clear I2C_DISABLE bits for PRM_VOLTCTRL register and the
device reboots if I2C_DISABLE bits are cleared.

Let's configure the voltage controller i2c for low-speed mode as done in
the Motorola Mapphone Android Linux kernel. This saves us about 7mW of
power during retention compared to the high-speed values.

Let's also change the low-speed warning to pr_info about relying on the
bootloader configured low-speed values like we currently do.

Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>