[ARM] 4863/1: AT91: CAP9 USART definitions for early debug

Define AT91_USART0, 1, 2 so the selection of the UART for early kernel
messages works.
   (See commit fa3218d8

).

Replace AT91_SHDC with AT91_SHDWC to be consistent with other AT91 platforms.

Signed-off-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>