Semihosting: Fix file mode to load binaries on Windows

Trusted firmware binaries loaded via semihosting used to be
opened using 'r' mode (i.e. read mode).  This is fine on POSIX
conforming systems (including Linux) but for Windows it also means
that the file should be opened in text mode. 'rb' mode must be
specified instead for binary mode.  On POSIX conforming systems,
'rb' mode is equivalent to 'r' mode so it does no harm.

Fixes ARM-software/tf-issues#69

Change-Id: Ifa53f2ecfd765f572dea5dd73191f9fe2b2c2011