[otbn] Indent loop body in assembly
[otbn] Indent loop body in assembly

Comsmetic change: Indent the loop body in assembly code to make it
easier to read.

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

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