xen/arm: Rename vgic_reg* functions definitions and calls to vreg_reg*

xen/arm: Rename vgic_reg* functions definitions and calls to vreg_reg*

This patch renames the vgic_reg* access functions defined in vreg.h to vreg_reg*
and replaces all calls to vgic_reg* functions in vgic/its emulation code to vreg_reg*.

vreg_reg* are generic functions, which can be used to operate on 32/64-bit registers.

SBSA UART emulation code will also use vreg_reg* access functions for
accessing emulated pl011 registers.

Signed-off-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>