ARC: HSDK: Add platform-specific commands

This patch add support of hsdk platform-specific commands:

hsdk_clock set - set clock from axi_freq, cpu_freq and tun_freq
environment variables/command line arguments

hsdk_clock get - save clock frequencies to axi_freq, cpu_freq
and tun_freq environment variables

hsdk_clock print - show CPU, AXI, DDR and TUNNEL current
clock frequencies.

hsdk_clock print_all - show all currently used clock frequencies.

hsdk_init - setup board HW in one of pre-defined configuration
(hsdk_hs34 / hsdk_hs36 / hsdk_hs36_ccm / hsdk_hs38 /
hsdk_hs38_ccm / hsdk_hs38x2 / hsdk_hs38x3 / hsdk_hs38x4)

hsdk_go - run baremetal application on hsdk configured
by hsdk_init command.

This patch changes default behaviour of 'bootm' command:
now we are able to set number of CPUs to be kicked by setting
'core_mask' environment variable before 'bootm' command run.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>