staging: emxx_udc: Remove EMMA Mobile USB Gadget driver

No one stepped up to complete the EMMA Mobile USB Gadget driver, bring
it up to non-staging standards, and convert it to device tree.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Link: https://lore.kernel.org/r/c7bc2c95458f9710e043cbedee4270dd41fcae29.1705932585.git.geert+renesas@glider.be
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>