arm: semihosting: staticize internal functions

The semihosting code exposes internal file handle handling
functions to read(), open(), close() and get the length of
a certain file handle.

However the code using it is only interested in either
reading and entire named file into memory or getting the
file length of a file referred by name. No file handles
are used.

Thus make the file handle code internal to this file by
removing these functions from the semihosting header file
and staticize them.

This gives us some freedom to rearrange the semihosting
code without affecting the external interface.

Cc: Darwin Rambo <drambo@broadcom.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Mark Hambleton <mark.hambleton@arm.com>
Cc: Tom Rini <trini@ti.com>
Acked-by: Steve Rae <srae@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>