tty: serial: cpm_uart: Convert to use GPIO descriptors

The CPM UART (PowerPC) has an open coded GPIO modem control
handling. Since I can't test this I can't just migrate it to
the serial mctrl GPIO helper library though I wish I could.
I do second best and convert it to GPIO descriptors at least.

Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200229231842.247563-1-linus.walleij@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>