UART4 low level debug support. This helps in debugging with UART4
serial console output on DRA7 based platforms.

Extending the following fix for UART4.
commit 1c7e36bf ("ARM: DRA7: hwmod: Fix boot crash with DEBUG_LL enabled on UART3")

For using DEBUG_LL, enable CONFIG_DEBUG_OMAP4UART4 in menuconfig.
On DRA7, UART4 hwmod doesn't have this flag enabled,
failure observed when UART4 is used for low level debugging.

Hence, Enable DEBUG_OMAP4UART4_FLAGS for UART4 hwmod.
Signed-off-by: J.D. Schroeder <jay.schroeder@garmin.com>
Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>