MA-14318-1 Support dual bootloader for xen

MA-14318-1 Support dual bootloader for xen

Trusty is not supported for xen so we don't need to check
the keyslot package or rollback index in spl. Reassign the
dram address for spl and u-boot to avoid conflicts.

Support serial init functions to enable debug console
in spl when xen is running.

Test: Boot and A/B slot switch on imx8qm_mek.

Change-Id: If6829252f1ec2e32255f951715c8747181951fd0
Signed-off-by: Ji Luo <ji.luo@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit 30beee3fe6d4b35166c6e17203909231d33bc2db)
(cherry picked from commit a4ffa905d90d14ccdd146e4b38a69d40a568b747)
(cherry picked from commit ca5e676b7a7c5ea7b435cb05eba180f3a25e5335)