bootm: Reduce arguments to boot_get_loadables()

bootm: Reduce arguments to boot_get_loadables()

This function only uses two arguments. The 'arch' always has a constant
value, so drop it. This simplifies the function call.

Tidy up the function comment while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>