[POWERPC] Fix cpm_uart driver

in cpm_uart_cpm1.h, DPRAM_BASE is assigned an address derived from cpmp.
On ARC=ppc, this is a physical address with 1:1 DMA mapping which can't
be used for arithmetric compare operations with virtual addresses
returned by cpm_dpram_addr. This patch changes the assignment to use
cpm_dpram_addr as well, like in cpm_uart_cpm2.h.

Signed-off-by: Jochen Friedrich <jochen@scram.de>