xtensa: fix boot parameters parsing

Boot parameter tags with handlers are ignored like this:

	[    0.000000] Ignoring tag 0x00001003
	[    0.000000] Ignoring tag 0x00001001
	[    0.000000] Ignoring tag 0x00001004

because neither tagtable entries nor tag handlers appear in the vmlinux.

Fix tagtable definition attributes so that tag entries are not dropped.

Fix end of memory bank calculation in parse_tag_mem: it is intended to
round down to page size, but instead did something strange leading to
hang right after boot.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>