[ARM] OMAP2/3 clockdomains: combine pwrdm, pwrdm_name into union in struct clockdomain

struct clockdomain contains a struct powerdomain *pwrdm and const char
*pwrdm_name.  The pwrdm_name is only used at initialization to look up
the appropriate pwrdm pointer.  Combining these into a union saves
about 100 bytes on 3430SDP.  This patch should not cause any change in
kernel function.

Updated to gracefully handle autodeps that contain invalid powerdomains,
per Russell King's review comments.

Boot-tested on BeagleBoard ES2.1.

linux-omap source commit is 718fc6cd4db902aa2242a736cc3feb8744a4c71a.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>