arm: vsmc: remove 64 bit mode check in PSCI handler

arm: vsmc: remove 64 bit mode check in PSCI handler

PSCI handling code had helper routine that checked calling convention.
It does not needed anymore, because:

 - Generic handler checks that 64 bit calls can be made only by
   64 bit guests.

 - SMCCC requires that 64-bit handler should support both 32 and 64 bit
   calls even if they originate from 64 bit caller.

This patch removes that extra check.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>