arm: semihosting: get rid of forward declarations

By rearranging the functions in the semihosting code we can
avoid forward-declaration of the internal static functions.
This puts the stuff in a logical order: read/open/close/len
and then higher-order functions follow at the end.

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>