x86/platform/uv: Unexport symbols only used by x2apic_uv_x.c

uv_bios_set_legacy_vga_target, uv_bios_freq_base, uv_bios_get_sn_info,
uv_type, system_serial_number and sn_region_size are only used in
x2apic_uv_x.c, which can't be modular.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Not-acked-by: Dimitri Sivanich <sivanich@hpe.com>
Cc: Russ Anderson <rja@hpe.com>
Link: https://lkml.kernel.org/r/20200504171527.2845224-5-hch@lst.de