[PARISC] Clean up asm-parisc/serial.h

Russell King pointed out that asm/serial.h is anachronistic and we were
misusing BASE_BAUD.  So fix BASE_BAUD for PCI 16550 UARTs, move LASI_BASE_BAUD
into 8250_gsc, and fix the obsolete comment about reserving serial port slots.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>