Docs: Refine tfm_sw_requirement.rst

Simplify the tfm software requirement document and change the document
to 'tfm_getting_started.rst', let it more
suitable for new starter.

Signed-off-by: Summer Qin <summer.qin@arm.com>
Change-Id: I10379aa92ac2c6cd784f0fd1b1b5431243716d74
11 files changed