ARM: dts: am33xx: Add spi alias to match SOC schematics

Linux bus numbers should match the numbers defined by the chip
manufacturer. This patch add's spi aliases to achieve that bus
naming convention.

Signed-off-by: Suniel Mahesh <sunil.m@techveda.org>
Signed-off-by: Karthik Tummala <karthik@techveda.org>
Tested-by: Karthik Tummala <karthik@techveda.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>