Rename PLAT_ARM_BL31_RUN_UART* variable

The variable is renamed to PLAT_ARM_RUN_UART as
the UART is used outside BL31 as well.

Change-Id: I00e3639dfb2001758b7d24548c11236c6335f64a
Signed-off-by: Usama Arif <usama.arif@arm.com>