fix(arch.h): fix TCR_EL2_TG1_4K constant value

TCR_EL2_TG1_4K is defined as 0x0, which is a reserved value.
This patch redefines it to 0x2, which is the right one.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: Ife9bd2c0630199b4811cc56c091eb0d9df985ff5