selftests/arm64: Fix grammatical error in string literals

Fix grammatical error in <past tense verb> + <infinitive>
construct related to memory allocation checks.
In essence change "Failed to allocated" to "Failed to allocate".

Signed-off-by: Nikola Z. Ivanov <zlatistiv@gmail.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>