ARM: rpi: add some missing Kconfig help text

Add notes re: enabling the UART to the RPi 3 32-bit help text. Fully
describe the RPi 3 64-bit board option.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
1 file changed