TI: DaVinci DM646x: Update flag used to represent DM646x SOC's

In the DaVinci specific code, we use both CONFIG_SOC_DM646X and
CONFIG_SOC_DM646x to represent DM646x specific code.
This patch changes occurrences of CONFIG_SOC_DM646x to
CONFIG_SOC_DM646X. This is because for DM644x series of SOCs we use
the flag CONFIG_SOC_DM644X. We want some uniformity.

Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>
Acked-by: Tom Rix <Tom.Rix@windriver.com>