mm: Override mTHP "enabled" defaults at kernel cmdline

Add thp_anon= cmdline parameter to allow specifying the default
enablement of each supported anon THP size. The parameter accepts the
following format and can be provided multiple times to configure each
size:

thp_anon=<size>[KMG]:<value>

See Documentation/admin-guide/mm/transhuge.rst for more details.

Configuring the defaults at boot time is useful to allow early user
space to take advantage of mTHP before its been configured through
sysfs.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>