serial: ucc_uart: stub out soft_uart_init for !CONFIG_PPC32

The Soft UART hack is only needed for some PPC-based SOCs. To allow
building this driver for non-PPC, guard soft_uart_init() and its
helpers by CONFIG_PPC32, and use a no-op soft_uart_init() otherwise.

Reviewed-by: Timur Tabi <timur@kernel.org>
Acked-by: Timur Tabi <timur@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Li Yang <leoyang.li@nxp.com>