cpm_uart: Don't use alloc_bootmem in cpm_uart_cpm2.c

This is another alloc_bootmem() -> kzalloc() change, this time to
fix the non-fatal badness caused when booting with a cpm2_uart console.

Signed-off-by: Mark Ware <mware@elphinstone.net>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>