staging: sm750fb: Merge ddk750_help.* into ddk750_chip.*.

The file ddk750_help.c contained only one function declaration,
so it was merged into ddk750_chip.c to simplify the driver.

Also, ddk750_help.h was merged into ddk750_chip.h to keep consistency.

With these changes a few global variables are removed and the function
ddk750_set_mmio is rewritten, so its purpose in the code is clearer.

Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>