coresight: explicitly control ETMCR & preserve prog & power down bits

Explicitly clear and set ETMCR power down bit as part of the ETM
register programming sequence since if this bit is set, writes to
some registers and fields might be ignored.

Moreover, ETMCR prog and power down bits are not directly modifiable
via the sysfs based MODE selction code. Instead they can implicitly get
modified by other code. So always preserve their values while
enabling ETM trace since the ctrl is a shadow of ETMCR bits that
are only modifiable via the sysfs based MODE selection code.

Change-Id: I842a42acdafb112759a1787ec6fc41140812020d
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>