arm64: add support for MSM HSL earlyprintk

Add support for arm64 earlyprintk using the MSM UARTDM
hardware block. Such support can be enabled by passing
something like "earlyprintk=msm_hsl_uart,0xf991e000" (or
the appropriate equivalent) on the kernel command line.

Change-Id: Ia43ad430e8bdcb0fd724c60949b82b256e2611a8
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>