arm64/mm: Detect and enable FEAT_LPA2

Detect FEAT_LPA2 implementation early enough during boot when requested via
CONFIG_ARM64_PA_BITS_52_LPA2 and remember in a variable arm64_lpa2_enabled.
This variable could then be used to turn on TCR_EL1.TCR_DS effecting the 52
bits PA range or fall back to default 48 bits PA range if FEAT_LPA2 feature
was requested but found not to be implemented.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>