[MIPS] Alchemy: move UART platform code to its proper place

Move the code registering the Alchemy UART platform devices from
drivers/serial/ to its proper place, into the Alchemy platform code.  Fix
the related Kconfig entry, while at it...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>