yocto/fvp-rd-kronos-extras: Increase Linux boot timeout

Increase TEST_FVP_LINUX_BOOT_TIMEOUT because increasing the
RSS clock frequency causes the primary compute to slow down
during the linux boot while the RSS is performing the
necessary computations especially in the PSA Crypto use case.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>