ARM: debug-ll: fix BCM63xx entry for multiplatform

During my randconfig build testing, I found that a kernel with
DEBUG_AT91_UART and ARCH_BCM_63XX fails to build:

arch/arm/include/debug/at91.S:18:0: error: "CONFIG_DEBUG_UART_VIRT" redefined [-Werror]

It turns out that the DEBUG_UART_BCM63XX option is enabled whenever
the ARCH_BCM_63XX is, and that breaks multiplatform kernels because
we then end up using the UART address from BCM63XX rather than the
one we actually configured (if any).

This changes the BCM63XX options to only have one Kconfig option,
and only enable that if the user explicitly turns it on.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b51312be ("ARM: BCM63XX: add low-level UART debug support")
Cc: stable@vger.kernel.org