ARM: 7860/1: debug: msm: Add DEBUG_LL support for ARCH_MSM8974

Add debug uart support for MSM8974. This patch adds a Kconfig
entry and the base address for the debug uart.

Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>