[DOCS] Use bootm for booting Xvisor on Raspberry Pi board

We should not be using u-boot 'go' command to launch Xvisor on
Raspberry Pi board because 'go' command does not disable interrupts
and mmu before jumping to Xvisor.

This patch updates documentation for booting Xvisor on raspberry pi
using u-boot 'bootm' command.

Signed-off-by: Anup Patel <anup@brainfault.org>