[otbn] P256: Explicitly align variables
[otbn] P256: Explicitly align variables

The variables that exist in p256.s need to be either 32b or 256b aligned
to ensure OTBN can access them. By "luck", the 256b variables at the end
of the file were 256b-aligned. But adding another variable in between
will break things, so let's be explicit.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan